diff --git a/Gruntfile.js b/Gruntfile.js
index e159dd03..850bb9e0 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -60,6 +60,7 @@ module.exports = function(grunt) {
             "src/drawerbase.js",
             "src/htmldrawer.js",
             "src/context2ddrawer.js",
+            "src/webgldrawer.js",
             "src/viewport.js",
             "src/tiledimage.js",
             "src/tilecache.js",
diff --git a/src/context2ddrawer.js b/src/context2ddrawer.js
index 20133570..0b0c2cec 100644
--- a/src/context2ddrawer.js
+++ b/src/context2ddrawer.js
@@ -108,9 +108,6 @@ class Context2dDrawer extends $.DrawerBase{
                 // _this._updateViewportWithTiledImage(tiledImage);
                 _this._drawTiles(tiledImage);
             }
-            else {
-                tiledImage._needsDraw = false;
-            }
         });
     }
 
@@ -371,11 +368,10 @@ class Context2dDrawer extends $.DrawerBase{
         }
 
         // Iterate over the tiles to draw, and draw them
-        for (var i = lastDrawn.length - 1; i >= 0; i--) {
+        for (var i = 0; i < lastDrawn.length; i++) {
             tile = lastDrawn[ i ];
-            this._drawTile( tile, tiledImage._drawingHandler, useSketch, sketchScale,
+            this._drawTile( tile, tiledImage, useSketch, sketchScale,
                 sketchTranslate, shouldRoundPositionAndSize, tiledImage.source );
-            tile.beingDrawn = true;
 
             if( this.viewer ){
                 /**
@@ -455,6 +451,24 @@ class Context2dDrawer extends $.DrawerBase{
         this._drawDebugInfo( tiledImage, lastDrawn );
     }
 
+    /**
+         * @private
+         * @inner
+         * This function converts the given point from to the drawer coordinate by
+         * multiplying it with the pixel density.
+         * This function does not take rotation into account, thus assuming provided
+         * point is at 0 degree.
+         * @param {OpenSeadragon.Point} point - the pixel point to convert
+         * @returns {OpenSeadragon.Point} Point in drawer coordinate system.
+         */
+    _viewportCoordToDrawerCoord(point) {
+        var vpPoint = this.viewport.pixelFromPointNoRotate(point, true);
+        return new $.Point(
+            vpPoint.x * $.pixelDensityRatio,
+            vpPoint.y * $.pixelDensityRatio
+        );
+    }
+
     /**
      * @private
      * @inner
@@ -466,7 +480,7 @@ class Context2dDrawer extends $.DrawerBase{
             for ( var i = lastDrawn.length - 1; i >= 0; i-- ) {
                 var tile = lastDrawn[ i ];
                 try {
-                    this.drawDebugInfo(tile, lastDrawn.length, i, tiledImage);
+                    this._drawDebugInfoOnTile(tile, lastDrawn.length, i, tiledImage);
                 } catch(e) {
                     $.console.error(e);
                 }
@@ -500,8 +514,7 @@ class Context2dDrawer extends $.DrawerBase{
      * @inner
      * Draws the given tile.
      * @param {OpenSeadragon.Tile} tile - The tile to draw.
-     * @param {Function} drawingHandler - Method for firing the drawing event if using canvas.
-     * drawingHandler({context, tile, rendered})
+     * @param {OpenSeadragon.TiledImage} tiledImage - The tiled image being drawn.
      * @param {Boolean} useSketch - Whether to use the sketch canvas or not.
      * where <code>rendered</code> is the context with the pre-drawn image.
      * @param {Float} [scale=1] - Apply a scale to tile position and size. Defaults to 1.
@@ -511,13 +524,13 @@ class Context2dDrawer extends $.DrawerBase{
      * context.
      * @param {OpenSeadragon.TileSource} source - The source specification of the tile.
      */
-    _drawTile( tile, drawingHandler, useSketch, scale, translate, shouldRoundPositionAndSize, source) {
+    _drawTile( tile, tiledImage, useSketch, scale, translate, shouldRoundPositionAndSize, source) {
         $.console.assert(tile, '[Drawer._drawTile] tile is required');
-        $.console.assert(drawingHandler, '[Drawer._drawTile] drawingHandler is required');
+        $.console.assert(tiledImage, '[Drawer._drawTile] drawingHandler is required');
 
         var context = this._getContext(useSketch);
         scale = scale || 1;
-        this._drawTileToCanvas(tile, context, drawingHandler, scale, translate, shouldRoundPositionAndSize, source);
+        this._drawTileToCanvas(tile, context, tiledImage, scale, translate, shouldRoundPositionAndSize, source);
 
     }
 
@@ -528,7 +541,7 @@ class Context2dDrawer extends $.DrawerBase{
      * @function
      * @param {OpenSeadragon.Tile} tile - the tile to draw to the canvas
      * @param {Canvas} context
-     * @param {Function} drawingHandler - Method for firing the drawing event.
+     * @param {OpenSeadragon.TiledImage} tiledImage - Method for firing the drawing event.
      * drawingHandler({context, tile, rendered})
      * where <code>rendered</code> is the context with the pre-drawn image.
      * @param {Number} [scale=1] - Apply a scale to position and size
@@ -538,7 +551,7 @@ class Context2dDrawer extends $.DrawerBase{
      * context.
      * @param {OpenSeadragon.TileSource} source - The source specification of the tile.
      */
-    _drawTileToCanvas( tile, context, drawingHandler, scale, translate, shouldRoundPositionAndSize, source) {
+    _drawTileToCanvas( tile, context, tiledImage, scale, translate, shouldRoundPositionAndSize, source) {
 
         var position = tile.position.times($.pixelDensityRatio),
             size     = tile.size.times($.pixelDensityRatio),
@@ -599,9 +612,7 @@ class Context2dDrawer extends $.DrawerBase{
             );
         }
 
-        // This gives the application a chance to make image manipulation
-        // changes as we are rendering the image
-        drawingHandler({context: context, tile: tile, rendered: rendered});
+        this._raiseTileDrawingEvent(tiledImage, context, tile, rendered);
 
         var sourceWidth, sourceHeight;
         if (tile.sourceBounds) {
@@ -805,7 +816,7 @@ class Context2dDrawer extends $.DrawerBase{
     }
 
     // private
-    drawDebugInfo(tile, count, i, tiledImage) {
+    _drawDebugInfoOnTile(tile, count, i, tiledImage) {
 
         var colorIndex = this.viewer.world.getIndexOfItem(tiledImage) % this.debugGridColor.length;
         var context = this.context;
diff --git a/src/drawerbase.js b/src/drawerbase.js
index fd8e94f8..93dbcf1b 100644
--- a/src/drawerbase.js
+++ b/src/drawerbase.js
@@ -90,6 +90,7 @@ $.DrawerBase = class DrawerBase{
         this.container  = $.getElement( options.element );
 
         // TO DO: Does this need to be in DrawerBase, or only in Drawer implementations?
+        // Original commment:
         // We force our container to ltr because our drawing math doesn't work in rtl.
         // This issue only affects our canvas renderer, but we do it always for consistency.
         // Note that this means overlays you want to be rtl need to be explicitly set to rtl.
@@ -199,6 +200,9 @@ $.DrawerBase = class DrawerBase{
      * placeholder methods are still in place.
      */
     _checkForAPIOverrides(){
+        if(this.createDrawingElement === $.DrawerBase.prototype.createDrawingElement){
+            throw("[drawer].createDrawingElement must be implemented by child class");
+        }
         if(this.draw === $.DrawerBase.prototype.draw){
             throw("[drawer].draw must be implemented by child class");
         }
@@ -208,12 +212,38 @@ $.DrawerBase = class DrawerBase{
         if(this.destroy === $.DrawerBase.prototype.destroy){
             throw("[drawer].destroy must be implemented by child class");
         }
-
         if(this.setImageSmoothingEnabled === $.DrawerBase.prototype.setImageSmoothingEnabled){
             throw("[drawer].setImageSmoothingEnabled must be implemented by child class");
         }
     }
 
+    _raiseTileDrawingEvent(tiledImage, context, tile, rendered){
+        /**
+         * This event is fired just before the tile is drawn giving the application a chance to alter the image.
+         *
+         * NOTE: This event is only fired in certain drawing contexts: either the 'context2d' drawer is
+         * being used, or the 'webgl' drawer with 'drawerOptions.webgl.continuousTileRefresh'.
+         *
+         * @event tile-drawing
+         * @memberof OpenSeadragon.Viewer
+         * @type {object}
+         * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised the event.
+         * @property {OpenSeadragon.Tile} tile - The Tile being drawn.
+         * @property {OpenSeadragon.TiledImage} tiledImage - Which TiledImage is being drawn.
+         * @property {CanvasRenderingContext2D} context - The HTML canvas context being drawn into.
+         * @property {CanvasRenderingContext2D} rendered - The HTML canvas context containing the tile imagery.
+         * @property {?Object} userData - Arbitrary subscriber-defined object.
+         */
+        this.viewer.raiseEvent('tile-drawing', {
+            tiledImage: tiledImage,
+            context: context,
+            tile: tile,
+            rendered: rendered
+        });
+    }
+
+
+
 
     // Utility functions
 
diff --git a/src/htmldrawer.js b/src/htmldrawer.js
index 7759781c..43759633 100644
--- a/src/htmldrawer.js
+++ b/src/htmldrawer.js
@@ -84,9 +84,6 @@ class HTMLDrawer extends $.DrawerBase{
             if (tiledImage.opacity !== 0 || tiledImage._preload) {
                 _this._drawTiles(tiledImage);
             }
-            else {
-                tiledImage._needsDraw = false;
-            }
         });
 
     }
@@ -145,7 +142,6 @@ class HTMLDrawer extends $.DrawerBase{
         for (var i = lastDrawn.length - 1; i >= 0; i--) {
             var tile = lastDrawn[ i ];
             this._drawTile( tile );
-            tile.beingDrawn = true;
 
             if( this.viewer ){
                 /**
diff --git a/src/imagetilesource.js b/src/imagetilesource.js
index 1ed4f85c..4596b2c0 100644
--- a/src/imagetilesource.js
+++ b/src/imagetilesource.js
@@ -196,8 +196,8 @@
          * Destroys ImageTileSource
          * @function
          */
-        destroy: function () {
-            this._freeupCanvasMemory();
+        destroy: function (viewer) {
+            this._freeupCanvasMemory(viewer);
         },
 
         // private
@@ -267,11 +267,23 @@
          * and Safari keeps canvas until its height and width will be set to 0).
          * @function
          */
-        _freeupCanvasMemory: function () {
+        _freeupCanvasMemory: function (viewer) {
             for (var i = 0; i < this.levels.length; i++) {
                 if(this.levels[i].context2D){
                     this.levels[i].context2D.canvas.height = 0;
                     this.levels[i].context2D.canvas.width = 0;
+
+                    /**
+                     * Triggered when an image has just been unloaded
+                     *
+                     * @event image-unloaded
+                     * @memberof OpenSeadragon.Viewer
+                     * @type {object}
+                     * @property {CanvasRenderingContext2D} context2D - The context that is being unloaded
+                     */
+                    viewer.raiseEvent("image-unloaded", {
+                        context2D: this.levels[i].context2D
+                    });
                 }
             }
         },
diff --git a/src/navigator.js b/src/navigator.js
index 2013d83d..21544b9a 100644
--- a/src/navigator.js
+++ b/src/navigator.js
@@ -170,9 +170,6 @@ $.Navigator = function( options ){
         style.border        = borderWidth + 'px solid ' + options.displayRegionColor;
         style.margin        = '0px';
         style.padding       = '0px';
-        //TODO: IE doesn't like this property being set
-        //try{ style.outline  = '2px auto #909'; }catch(e){/*ignore*/}
-
         style.background    = 'transparent';
 
         // We use square bracket notation on the statement below, because float is a keyword.
diff --git a/src/openseadragon.js b/src/openseadragon.js
index 7f3858cd..1e6f1d75 100644
--- a/src/openseadragon.js
+++ b/src/openseadragon.js
@@ -190,15 +190,15 @@
   *     Zoom level to use when image is first opened or the home button is clicked.
   *     If 0, adjusts to fit viewer.
   *
-  * @property {String|DrawerImplementation|Array} [drawer = ['context2d', 'html']]
+  * @property {String|DrawerImplementation|Array} [drawer = ['webgl', 'context2d', 'html']]
   *     Which drawer to use. Valid strings are 'context2d' and 'html'. Valid drawer
   *     implementations are constructors of classes that extend OpenSeadragon.DrawerBase.
   *     An array of strings and/or constructors can be used to indicate the priority
   *     of different implementations, which will be tried in order based on browser support.
   *
-  * @property {Object} [drawerOptions = {}]
-  *     Options to pass to the selected drawer implementation. See documentation
-  *     for Drawer classes that extend DrawerBase for further information.
+  * @property {Object} drawerOptions
+  *     Options to pass to the selected drawer implementation. For details
+  *     please @see {@link drawerOptions}.
   *
   * @property {Number} [opacity=1]
   *     Default proportional opacity of the tiled images (1=opaque, 0=hidden)
@@ -1346,9 +1346,32 @@ function OpenSeadragon( options ){
             compositeOperation:                null, // to be passed into each TiledImage
 
             // DRAWER SETTINGS
-            drawer:                            ['context2d', 'html'], // prefer using canvas, fallback to html
-            drawerOptions:                     {},
+            drawer:                            ['webgl', 'context2d', 'html'], // prefer using webgl, context2d, fallback to html
             useCanvas:                         true,  // deprecated - set drawer and drawerOptions
+                /**
+                 * drawerOptions dictionary.
+                 * @type {Object} drawerOptions
+                 * @property {Object} webgl - options if the WebGLDrawer is used.
+                 * Set 'continuousTileFresh: true' if tile data is modified programmatically
+                 * by filtering plugins or similar.
+                 * @property {Object} context2d - options if the Context2dDrawer is used
+                 * @property {Object} html - options if the HTMLDrawer is used
+                 * @property {Object} custom - options if a custom drawer is used
+                 */
+            drawerOptions: {
+                webgl: {
+                    continuousTileRefresh: false,
+                },
+                context2d: {
+
+                },
+                html: {
+
+                },
+                custom: {
+
+                }
+            },
 
             // TILED IMAGE SETTINGS
             preload:                           false, // to be passed into each TiledImage
diff --git a/src/tile.js b/src/tile.js
index 682236fb..e95633a1 100644
--- a/src/tile.js
+++ b/src/tile.js
@@ -81,6 +81,12 @@
          * @memberof OpenSeadragon.Tile#
          */
         this.bounds  = bounds;
+        /**
+         * Where this tile fits, in normalized coordinates, after positioning
+         * @member {OpenSeadragon.Rect} positionedBounds
+         * @memberof OpenSeadragon.Tile#
+         */
+        this.positionedBounds  = new OpenSeadragon.Rect(bounds.x, bounds.y, bounds.width, bounds.height);
         /**
          * The portion of the tile to use as the source of the drawing operation, in pixels. Note that
          * this only works when drawing with canvas; when drawing with HTML the entire tile is always used.
@@ -324,7 +330,7 @@
          * @returns {CanvasRenderingContext2D}
          */
         getCanvasContext: function() {
-            return this.context2D || this.cacheImageRecord.getRenderedContext();
+            return this.context2D || (this.cacheImageRecord && this.cacheImageRecord.getRenderedContext());
         },
 
         /**
diff --git a/src/tilecache.js b/src/tilecache.js
index d890b8a8..7d9e5478 100644
--- a/src/tilecache.js
+++ b/src/tilecache.js
@@ -236,19 +236,50 @@ $.TileCache.prototype = {
         var tile = tileRecord.tile;
         var tiledImage = tileRecord.tiledImage;
 
+        // tile.getCanvasContext should always exist in normal usage (with $.Tile)
+        // but the tile cache test passes in a dummy object
+        let context2D = tile.getCanvasContext && tile.getCanvasContext();
+
         tile.unload();
         tile.cacheImageRecord = null;
 
         var imageRecord = this._imagesLoaded[tile.cacheKey];
+        if(!imageRecord){
+            return;
+        }
         imageRecord.removeTile(tile);
         if (!imageRecord.getTileCount()) {
+
             imageRecord.destroy();
             delete this._imagesLoaded[tile.cacheKey];
             this._imagesLoadedCount--;
+
+            if(context2D){
+                /**
+                 * Free up canvas memory
+                 * (iOS 12 or higher on 2GB RAM device has only 224MB canvas memory,
+                 * and Safari keeps canvas until its height and width will be set to 0).
+                 */
+                context2D.canvas.width = 0;
+                context2D.canvas.height = 0;
+
+                /**
+                 * Triggered when an image has just been unloaded
+                 *
+                 * @event image-unloaded
+                 * @memberof OpenSeadragon.Viewer
+                 * @type {object}
+                 * @property {CanvasRenderingContext2D} context2D - The context that is being unloaded
+                 */
+                tiledImage.viewer.raiseEvent("image-unloaded", {
+                    context2D: context2D
+                });
+            }
+
         }
 
         /**
-         * Triggered when a tile has just been unloaded from memory.
+         * Triggered when a tile has just been unloaded from the cache.
          *
          * @event tile-unloaded
          * @memberof OpenSeadragon.Viewer
@@ -260,6 +291,7 @@ $.TileCache.prototype = {
             tile: tile,
             tiledImage: tiledImage
         });
+
     }
 };
 
diff --git a/src/tiledimage.js b/src/tiledimage.js
index a2d6c34c..fceb30ae 100644
--- a/src/tiledimage.js
+++ b/src/tiledimage.js
@@ -85,7 +85,7 @@
  *      A set of headers to include when making tile AJAX requests.
  */
 $.TiledImage = function( options ) {
-    var _this = this;
+    this._initialized = false;
     /**
      * The {@link OpenSeadragon.TileSource} that defines this TiledImage.
      * @member {OpenSeadragon.TileSource} source
@@ -162,7 +162,10 @@ $.TiledImage = function( options ) {
         _needsDraw:     true,  // Does the tiledImage need to update the viewport again?
         _hasOpaqueTile: false,  // Do we have even one fully opaque tile?
         _tilesLoading:  0,     // The number of pending tile requests.
-        _tilesToDraw:   [],    // info about the tiles currently in the viewport
+        _tilesToDraw:   [],    // info about the tiles currently in the viewport, two deep: array[level][tile]
+        _lastDrawn:     [],    // array of tiles that were last fetched by the drawer
+        _isBlending:    false, // Are any tiles still being blended?
+        _wasBlending:   false, // Were any tiles blending before the last draw?
         //configurable settings
         springStiffness:                   $.DEFAULT_SETTINGS.springStiffness,
         animationTime:                     $.DEFAULT_SETTINGS.animationTime,
@@ -220,30 +223,9 @@ $.TiledImage = function( options ) {
         this.fitBounds(fitBounds, fitBoundsPlacement, true);
     }
 
-    // We need a callback to give image manipulation a chance to happen
-    this._drawingHandler = function(args) {
-        /**
-         * This event is fired just before the tile is drawn giving the application a chance to alter the image.
-         *
-         * NOTE: This event is only fired when the drawer is using a &lt;canvas&gt;.
-         *
-         * @event tile-drawing
-         * @memberof OpenSeadragon.Viewer
-         * @type {object}
-         * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised the event.
-         * @property {OpenSeadragon.Tile} tile - The Tile being drawn.
-         * @property {OpenSeadragon.TiledImage} tiledImage - Which TiledImage is being drawn.
-         * @property {OpenSeadragon.Tile} context - The HTML canvas context being drawn into.
-         * @property {OpenSeadragon.Tile} rendered - The HTML canvas context containing the tile imagery.
-         * @property {?Object} userData - Arbitrary subscriber-defined object.
-         */
-        _this.viewer.raiseEvent('tile-drawing', $.extend({
-            tiledImage: _this
-        }, args));
-    };
-
     this._ownAjaxHeaders = {};
     this.setAjaxHeaders(ajaxHeaders, false);
+    this._initialized = true;
 };
 
 $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadragon.TiledImage.prototype */{
@@ -311,7 +293,6 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
 
         if (updated || viewportChanged || !this._fullyLoaded){
             let fullyLoadedFlag = this._updateLevelsForViewport();
-            this._updateTilesInViewport();
             this._setFullyLoaded(fullyLoadedFlag);
         }
 
@@ -328,10 +309,13 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
 
     /**
      * Mark this TiledImage as having been drawn, so that it will only be drawn
-     * again if something changes about the image
+     * again if something changes about the image. If the image is still blending,
+     * this will have no effect.
+     * @returns {Boolean} whether the item still needs to be drawn due to blending
      */
     setDrawn: function(){
-        this._needsDraw = false;
+        this._needsDraw = this._isBlending || this._wasBlending;
+        return this._needsDraw;
     },
 
     /**
@@ -341,7 +325,7 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         this.reset();
 
         if (this.source.destroy) {
-            this.source.destroy();
+            this.source.destroy(this.viewer);
         }
     },
 
@@ -539,7 +523,7 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
             imageX = imageX.x;
         }
 
-        var point = this._imageToViewportDelta(imageX, imageY);
+        var point = this._imageToViewportDelta(imageX, imageY, current);
         if (current) {
             point.x += this._xSpring.current.value;
             point.y += this._ySpring.current.value;
@@ -934,9 +918,39 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         return this._flipped;
     },
     set flipped(flipped){
+        let changed = this._flipped !== !!flipped;
         this._flipped = !!flipped;
-        this._needsDraw = true;
-        this._raiseBoundsChange();
+        if(changed){
+            this.update(true);
+            this._needsDraw = true;
+            this._raiseBoundsChange();
+        }
+    },
+
+    get wrapHorizontal(){
+        return this._wrapHorizontal;
+    },
+    set wrapHorizontal(wrap){
+        let changed = this._wrapHorizontal !== !!wrap;
+        this._wrapHorizontal = !!wrap;
+        if(this._initialized && changed){
+            this.update(true);
+            this._needsDraw = true;
+            // this._raiseBoundsChange();
+        }
+    },
+
+    get wrapVertical(){
+        return this._wrapVertical;
+    },
+    set wrapVertical(wrap){
+        let changed = this._wrapVertical !== !!wrap;
+        this._wrapVertical = !!wrap;
+        if(this._initialized && changed){
+            this.update(true);
+            this._needsDraw = true;
+            // this._raiseBoundsChange();
+        }
     },
 
     get debugMode(){
@@ -1058,7 +1072,20 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
      * @returns {Array} Array of Tiles that make up the current view
      */
     getTilesToDraw: function(){
-        return this._tilesToDraw;
+
+        let tileArray = this._tilesToDraw.flat();
+        // update all tiles (so blending can happen right at the time of drawing)
+        this._updateTilesInViewport(tileArray);
+        // _tilesToDraw might have been updated by the update; refresh it
+        tileArray = this._tilesToDraw.flat();
+
+         // mark the tiles as being drawn, so that they won't be discarded from
+        // the tileCache
+        tileArray.forEach(tileInfo => {
+            tileInfo.tile.beingDrawn = true;
+        });
+        this._lastDrawn = tileArray;
+        return tileArray;
     },
 
     /**
@@ -1289,7 +1316,7 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         var currentTime = $.now();
 
         // reset each tile's beingDrawn flag
-        this._tilesToDraw.forEach(tileinfo => {
+        this._lastDrawn.forEach(tileinfo => {
             tileinfo.tile.beingDrawn = false;
         });
         // clear the list of tiles to draw
@@ -1391,7 +1418,7 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
                 };
             })(level, levelOpacity, currentTime);
 
-            this._tilesToDraw = this._tilesToDraw.concat(tiles.map(makeTileInfoObject));
+            this._tilesToDraw[level] = tiles.map(makeTileInfoObject);
 
             // Stop the loop if lower-res tiles would all be covered by
             // already drawn tiles
@@ -1419,47 +1446,54 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
      * Update all tiles that contribute to the current view
      *
      */
-    _updateTilesInViewport: function() {
-        var _this = this;
+    _updateTilesInViewport: function(tiles) {
+        let currentTime = $.now();
+        let _this = this;
         this._tilesLoading = 0;
+        this._wasBlending = this._isBlending;
+        this._isBlending = false;
         this.loadingCoverage = {};
+        let lowestLevel = tiles.length ? tiles[0].level : 0;
 
-        var drawArea = this.getDrawArea();
+        let drawArea = this.getDrawArea();
         if(!drawArea){
             return;
         }
 
         function updateTile(info){
-            var tile = info.tile;
+            let tile = info.tile;
             if(tile && tile.loaded){
-                var needsDraw = _this._blendTile(
+                let tileIsBlending = _this._blendTile(
                     tile,
                     tile.x,
                     tile.y,
                     info.level,
                     info.levelOpacity,
-                    info.currentTime
+                    currentTime,
+                    lowestLevel
                 );
-                if(needsDraw){
-                    _this._needsDraw = true;
-                }
+                _this._isBlending = _this._isBlending || tileIsBlending;
+                _this._needsDraw = _this._needsDraw || tileIsBlending || this._wasBlending;
             }
         }
 
-        // Update each tile in the _tilesToDraw list. As the tiles are updated,
+        // Update each tile in the _lastDrawn list. As the tiles are updated,
         // the coverage provided is also updated. If a level provides coverage
         // as part of this process, discard tiles from lower levels
         let level = 0;
-        for(let i = 0; i < this._tilesToDraw.length; i++){
-            let tile = this._tilesToDraw[i];
+        for(let i = 0; i < tiles.length; i++){
+            let tile = tiles[i];
             updateTile(tile);
             if(this._providesCoverage(this.coverage, tile.level)){
                 level = Math.max(level, tile.level);
-                // break;
             }
         }
         if(level > 0){
-            this._tilesToDraw = this._tilesToDraw.filter(tile => tile.level >= level);
+            for( let levelKey in this._tilesToDraw ){
+                if( levelKey < level ){
+                    delete this._tilesToDraw[levelKey];
+                }
+            }
         }
 
     },
@@ -1478,10 +1512,11 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
      * @param {Number} level
      * @param {Number} levelOpacity
      * @param {Number} currentTime
-     * @returns {Boolean}
+     * @param {Boolean} lowestLevel
+     * @returns {Boolean} whether the opacity of this tile has changed
      */
-    _blendTile: function(tile, x, y, level, levelOpacity, currentTime ){
-        var blendTimeMillis = 1000 * this.blendTime,
+    _blendTile: function(tile, x, y, level, levelOpacity, currentTime, lowestLevel ){
+        let blendTimeMillis = 1000 * this.blendTime,
             deltaTime,
             opacity;
 
@@ -1492,20 +1527,23 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         deltaTime   = currentTime - tile.blendStart;
         opacity     = blendTimeMillis ? Math.min( 1, deltaTime / ( blendTimeMillis ) ) : 1;
 
+        // if this tile is at the lowest level being drawn, render at opacity=1
+        if(level === lowestLevel){
+            opacity = 1;
+            deltaTime = blendTimeMillis;
+        }
+
         if ( this.alwaysBlend ) {
             opacity *= levelOpacity;
         }
-
         tile.opacity = opacity;
 
         if ( opacity === 1 ) {
             this._setCoverage( this.coverage, level, x, y, true );
             this._hasOpaqueTile = true;
-        } else if ( deltaTime < blendTimeMillis ) {
-            return true;
         }
-
-        return false;
+        // return true if the tile is still blending
+        return deltaTime < blendTimeMillis;
     },
 
     /**
@@ -1647,6 +1685,11 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         boundsSize.x *= this._scaleSpring.current.value;
         boundsSize.y *= this._scaleSpring.current.value;
 
+        tile.positionedBounds.x = boundsTL.x;
+        tile.positionedBounds.y = boundsTL.y;
+        tile.positionedBounds.width = boundsSize.x;
+        tile.positionedBounds.height = boundsSize.y;
+
         var positionC = viewport.pixelFromPointNoRotate(boundsTL, true),
             positionT = viewport.pixelFromPointNoRotate(boundsTL, false),
             sizeC = viewport.deltaPixelsFromPointsNoRotate(boundsSize, true),
diff --git a/src/viewer.js b/src/viewer.js
index da334094..bcb7cc94 100644
--- a/src/viewer.js
+++ b/src/viewer.js
@@ -446,7 +446,7 @@ $.Viewer = function( options ) {
         delete this.drawerOptions.useCanvas;
     }
     let drawerPriority = Array.isArray(this.drawer) ? this.drawer : [this.drawer];
-    let drawersToTry = drawerPriority.filter(d => ['context2d', 'html'].includes(d) || (d.prototype && d.prototype.isOpenSeadragonDrawer) );
+    let drawersToTry = drawerPriority.filter(d => ['webgl', 'context2d', 'html'].includes(d) || (d.prototype && d.prototype.isOpenSeadragonDrawer) );
     if(drawerPriority.length !== drawersToTry.length){
         $.console.error('An invalid drawer was requested.');
     }
@@ -458,11 +458,19 @@ $.Viewer = function( options ) {
     this.drawer = null; // TO DO: how to deal with the possibility that none of the requested drawers are supported?
     for(let i = 0; i < drawersToTry.length; i++){
         let Drawer = drawersToTry[i];
+        let optsKey = null;
         // replace text-based option with appropriate constructor
         if (Drawer === 'context2d'){
             Drawer = $.Context2dDrawer;
+            optsKey = 'context2d';
         } else if (Drawer === 'html'){
             Drawer = $.HTMLDrawer;
+            optsKey = 'html';
+        } else if (Drawer === 'webgl'){
+            Drawer = $.WebGLDrawer;
+            optsKey = 'webgl';
+        } else {
+            optsKey = 'custom';
         }
         // if the drawer is supported, create it and break the loop
         if (Drawer.prototype.isSupported()){
@@ -471,7 +479,7 @@ $.Viewer = function( options ) {
                 viewport:           this.viewport,
                 element:            this.canvas,
                 debugGridColor:     this.debugGridColor,
-                options:            this.drawerOptions,
+                options:            this.drawerOptions[optsKey],
             });
             this.drawerOptions.constructor = Drawer;
             // TO DO: add an event that indicates which drawer was instantiated?
@@ -479,6 +487,10 @@ $.Viewer = function( options ) {
         }
         // TO DO: add an event that indicates that the selected drawer could not be created?
     }
+    if(this.drawer === null){
+        $.console.error('No drawer could be created!');
+        throw('Error with creating the selected drawer(s)');
+    }
 
 
     // Overlay container
@@ -1090,7 +1102,7 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
      * @returns {Boolean}
      */
     isFullPage: function () {
-        return THIS[ this.hash ].fullPage;
+        return THIS[this.hash] && THIS[ this.hash ].fullPage;
     },
 
 
@@ -1137,7 +1149,7 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
             return this;
         }
 
-        if ( fullPage ) {
+        if ( fullPage && this.element ) {
 
             this.elementSize = $.getElementSize( this.element );
             this.pageScroll = $.getPageScroll();
diff --git a/src/webgldrawer.js b/src/webgldrawer.js
new file mode 100644
index 00000000..9c9c7d33
--- /dev/null
+++ b/src/webgldrawer.js
@@ -0,0 +1,872 @@
+/*
+ * OpenSeadragon - WebGLDrawer
+ *
+ * Copyright (C) 2009 CodePlex Foundation
+ * Copyright (C) 2010-2023 OpenSeadragon contributors
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * - Redistributions of source code must retain the above copyright notice,
+ *   this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ *
+ * - Neither the name of CodePlex Foundation nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
+ * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+(function( $ ){
+
+ // internal class Mat3: implements matrix operations
+ // Modified from https://webglfundamentals.org/webgl/lessons/webgl-2d-matrices.html
+class Mat3{
+    constructor(values){
+        if(!values) {
+            values = [
+                0, 0, 0,
+                0, 0, 0,
+                0, 0, 0
+            ];
+        }
+
+        this.values = values;
+    }
+
+    static makeIdentity(){
+        return new Mat3([
+            1, 0, 0,
+            0, 1, 0,
+            0, 0, 1
+        ]);
+    }
+
+    static makeTranslation(tx, ty) {
+        return new Mat3([
+            1, 0, 0,
+            0, 1, 0,
+            tx, ty, 1,
+        ]);
+    }
+
+    static makeRotation(angleInRadians) {
+        var c = Math.cos(angleInRadians);
+        var s = Math.sin(angleInRadians);
+        return new Mat3([
+            c, -s, 0,
+            s, c, 0,
+            0, 0, 1,
+        ]);
+    }
+
+    static makeScaling(sx, sy) {
+        return new Mat3([
+            sx, 0, 0,
+            0, sy, 0,
+            0, 0, 1,
+        ]);
+    }
+
+    multiply(other) {
+        let a = this.values;
+        let b = other.values;
+
+        var a00 = a[0 * 3 + 0];
+        var a01 = a[0 * 3 + 1];
+        var a02 = a[0 * 3 + 2];
+        var a10 = a[1 * 3 + 0];
+        var a11 = a[1 * 3 + 1];
+        var a12 = a[1 * 3 + 2];
+        var a20 = a[2 * 3 + 0];
+        var a21 = a[2 * 3 + 1];
+        var a22 = a[2 * 3 + 2];
+        var b00 = b[0 * 3 + 0];
+        var b01 = b[0 * 3 + 1];
+        var b02 = b[0 * 3 + 2];
+        var b10 = b[1 * 3 + 0];
+        var b11 = b[1 * 3 + 1];
+        var b12 = b[1 * 3 + 2];
+        var b20 = b[2 * 3 + 0];
+        var b21 = b[2 * 3 + 1];
+        var b22 = b[2 * 3 + 2];
+        return new Mat3([
+            b00 * a00 + b01 * a10 + b02 * a20,
+            b00 * a01 + b01 * a11 + b02 * a21,
+            b00 * a02 + b01 * a12 + b02 * a22,
+            b10 * a00 + b11 * a10 + b12 * a20,
+            b10 * a01 + b11 * a11 + b12 * a21,
+            b10 * a02 + b11 * a12 + b12 * a22,
+            b20 * a00 + b21 * a10 + b22 * a20,
+            b20 * a01 + b21 * a11 + b22 * a21,
+            b20 * a02 + b21 * a12 + b22 * a22,
+        ]);
+    }
+
+}
+
+/**
+ * @class WebGLDrawer
+ * @memberof OpenSeadragon
+ * @classdesc Default implementation of WebGLDrawer for an {@link OpenSeadragon.Viewer}.
+ * @param {Object} options - Options for this Drawer.
+ * @param {OpenSeadragon.Viewer} options.viewer - The Viewer that owns this Drawer.
+ * @param {OpenSeadragon.Viewport} options.viewport - Reference to Viewer viewport.
+ * @param {Element} options.element - Parent element.
+ * @param {Number} [options.debugGridColor] - See debugGridColor in {@link OpenSeadragon.Options} for details.
+ */
+
+$.WebGLDrawer = class WebGLDrawer extends OpenSeadragon.DrawerBase{
+    constructor(options){
+        super(options);
+
+        this.destroyed = false;
+        // private members
+
+        this._TextureMap = new Map();
+        this._TileMap = new Map();
+
+        this._gl = null;
+        this._glLocs = null;
+        this._glProgram = null;
+        this._glPositionBuffer = null;
+        this._outputCanvas = null;
+        this._outputContext = null;
+        this._clippingCanvas = null;
+        this._clippingContext = null;
+        this._renderingCanvas = null;
+
+        // Add listeners for events that require modifying the scene or camera
+        this.viewer.addHandler("tile-ready", ev => this._tileReadyHandler(ev));
+        this.viewer.addHandler("image-unloaded", ev => this._imageUnloadedHandler(ev));
+
+        // this.viewer is set by parent constructor
+        // this.canvas is set by parent constructor, created and appended to the viewer container element
+        this._setupCanvases();
+
+        this._setupRenderer();
+
+        this.context = this._outputContext; // API required by tests
+    }
+
+    // Public API required by all Drawer implementations
+    /**
+     * Clean up the renderer, removing all resources
+     */
+    destroy(){
+        if(this.destroyed){
+            return;
+        }
+        // clear all resources used by the renderer, geometries, textures etc
+        let gl = this._gl;
+
+        // adapted from https://stackoverflow.com/a/23606581/1214731
+        var numTextureUnits = gl.getParameter(gl.MAX_TEXTURE_IMAGE_UNITS);
+        for (let unit = 0; unit < numTextureUnits; ++unit) {
+            gl.activeTexture(gl.TEXTURE0 + unit);
+            gl.bindTexture(gl.TEXTURE_2D, null);
+            gl.bindTexture(gl.TEXTURE_CUBE_MAP, null);
+        }
+        gl.bindBuffer(gl.ARRAY_BUFFER, null);
+        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);
+        gl.bindRenderbuffer(gl.RENDERBUFFER, null);
+        gl.bindFramebuffer(gl.FRAMEBUFFER, null);
+
+        let canvases = Array.from(this._TextureMap.keys());
+        canvases.forEach(canvas => {
+            this._cleanupImageData(canvas); // deletes texture, removes from _TextureMap
+        });
+
+        // Delete all our created resources
+        gl.deleteBuffer(this._glPositionBuffer);
+
+        // TO DO: if/when render buffers or frame buffers are used, release them:
+        // gl.deleteRenderbuffer(someRenderbuffer);
+        // gl.deleteFramebuffer(someFramebuffer);
+
+        // make canvases 1 x 1 px and delete references
+        this._renderingCanvas.width = this._renderingCanvas.height = 1;
+        this._clippingCanvas.width = this._clippingCanvas.height = 1;
+        this._outputCanvas.width = this._outputCanvas.height = 1;
+        this._renderingCanvas = null;
+        this._clippingCanvas = this._clippingContext = null;
+        this._outputCanvas = this._outputContext = null;
+
+        let ext = gl.getExtension('WEBGL_lose_context');
+        if(ext){
+            ext.loseContext();
+        }
+
+        // set our webgl context reference to null to enable garbage collection
+        this._gl = null;
+
+        // set our destroyed flag to true
+        this.destroyed = true;
+    }
+
+    // Public API required by all Drawer implementations
+    /**
+     *
+     * @returns true if the drawer supports rotation
+     */
+    canRotate(){
+        return true;
+    }
+
+    // Public API required by all Drawer implementations
+
+    /**
+     * @returns {Boolean} returns true if canvas and webgl are supported and
+     * three.js has been exposed as a global variable named THREE
+     */
+    isSupported(){
+        let canvasElement = document.createElement( 'canvas' );
+        let webglContext = $.isFunction( canvasElement.getContext ) &&
+                    canvasElement.getContext( 'webgl' );
+        let ext = webglContext.getExtension('WEBGL_lose_context');
+        if(ext){
+            ext.loseContext();
+        }
+        return !!( webglContext );
+    }
+
+    /**
+     * create the HTML element (canvas in this case) that the image will be drawn into
+     * @returns {Element} the canvas to draw into
+     */
+    createDrawingElement(){
+        let canvas = $.makeNeutralElement("canvas");
+        let viewportSize = this._calculateCanvasSize();
+        canvas.width = viewportSize.x;
+        canvas.height = viewportSize.y;
+        return canvas;
+    }
+
+    /**
+     *
+     * @param {Array} tiledImages Array of TiledImage objects to draw
+     */
+    draw(tiledImages){
+        let viewport = {
+            bounds: this.viewport.getBoundsNoRotate(true),
+            center: this.viewport.getCenter(true),
+            rotation: this.viewport.getRotation(true) * Math.PI / 180
+        };
+
+        let flipMultiplier = this.viewport.flipped ? -1 : 1;
+        // calculate view matrix for viewer
+        let posMatrix = Mat3.makeTranslation(-viewport.center.x, -viewport.center.y);
+        let scaleMatrix = Mat3.makeScaling(2 / viewport.bounds.width * flipMultiplier, -2 / viewport.bounds.height);
+        let rotMatrix = Mat3.makeRotation(-viewport.rotation);
+        let viewMatrix = scaleMatrix.multiply(rotMatrix).multiply(posMatrix);
+
+        //iterate over tiled imagesget the list of tiles to draw
+        this._outputContext.clearRect(0, 0, this._outputCanvas.width, this._outputCanvas.height);
+
+        // TO DO: further optimization is possible.
+        // If no clipping and no composite operation, the tiled images
+        // can all be drawn onto the rendering canvas at the same time, avoiding
+        // unnecessary clearing and copying of the pixel data.
+        // For now, I'm doing it this way to replicate full functionality
+        // of the context2d drawer
+        tiledImages.forEach( (tiledImage, i) => {
+            // clear the rendering canvas
+            this._gl.clear(this._gl.COLOR_BUFFER_BIT);
+
+            // set opacity for this image
+            this._gl.uniform1f(this._glLocs.uOpacityMultiplier, tiledImage.opacity);
+
+            //get the list of tiles to draw
+            let tilesToDraw = tiledImage.getTilesToDraw();
+
+            if(tilesToDraw.length === 0){
+                return;
+            }
+
+            let overallMatrix = viewMatrix;
+
+            let imageRotation = tiledImage.getRotation(true);
+            if( imageRotation % 360 !== 0){
+                let imageRotationMatrix = Mat3.makeRotation(-imageRotation * Math.PI / 180);
+                let imageCenter = tiledImage.getBoundsNoRotate(true).getCenter();
+                let t1 = Mat3.makeTranslation(imageCenter.x, imageCenter.y);
+                let t2 = Mat3.makeTranslation(-imageCenter.x, -imageCenter.y);
+
+                // update the view matrix to account for this image's rotation
+                let localMatrix = t1.multiply(imageRotationMatrix).multiply(t2);
+                overallMatrix = viewMatrix.multiply(localMatrix);
+            }
+
+            for(let i = 0; i < tilesToDraw.length; i++){
+                let tile = tilesToDraw[i].tile;
+                let texture = this._TextureMap.get(tile.getCanvasContext().canvas);
+                if(texture){
+                    this._drawTile(tile, tiledImage, texture, overallMatrix, tiledImage.opacity);
+                } else {
+                    // console.log('No tile info', tile);
+                }
+            }
+
+            // composite onto the output canvas, clipping if necessary
+            this._outputContext.save();
+
+            // set composite operation; ignore for first image drawn
+            this._outputContext.globalCompositeOperation = i === 0 ? null : tiledImage.compositeOperation || this.viewer.compositeOperation;
+            if(tiledImage._croppingPolygons || tiledImage._clip){
+                this._renderToClippingCanvas(tiledImage);
+                this._outputContext.drawImage(this._clippingCanvas, 0, 0);
+
+            } else {
+                this._outputContext.drawImage(this._renderingCanvas, 0, 0);
+            }
+            this._outputContext.restore();
+            if(tiledImage.debugMode){
+                let colorIndex = this.viewer.world.getIndexOfItem(tiledImage) % this.debugGridColor.length;
+                let strokeStyle = this.debugGridColor[colorIndex];
+                let fillStyle = this.debugGridColor[colorIndex];
+                this._drawDebugInfo(tilesToDraw, tiledImage, strokeStyle, fillStyle);
+            }
+
+            // TO DO: this is necessary for the tests to pass, but doesn't totally make sense for the webgl drawer.
+            // Iterate over the tiles that were just drawn and fire the tile-drawn event
+            for(let i = 0; i < tilesToDraw.length; i++){
+                let tile = tilesToDraw[i].tile;
+
+                if( this.viewer ){
+                    /**
+                     * Raised when a tile is drawn to the canvas
+                     *
+                     * @event tile-drawn
+                     * @memberof OpenSeadragon.Viewer
+                     * @type {object}
+                     * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised the event.
+                     * @property {OpenSeadragon.TiledImage} tiledImage - Which TiledImage is being drawn.
+                     * @property {OpenSeadragon.Tile} tile
+                     * @property {?Object} userData - Arbitrary subscriber-defined object.
+                     */
+                    this.viewer.raiseEvent( 'tile-drawn', {
+                        tiledImage: tiledImage,
+                        tile: tile
+                    });
+                }
+            }
+
+        });
+
+    }
+
+    // Public API required by all Drawer implementations
+    /**
+     * Set the context2d imageSmoothingEnabled parameter
+     * @param {Boolean} enabled
+     */
+    setImageSmoothingEnabled(enabled){
+        this._clippingContext.imageSmoothingEnabled = enabled;
+        this._outputContext.imageSmoothingEnabled = enabled;
+    }
+
+    /**
+     * Draw a rect onto the output canvas for debugging purposes
+     * @param {OpenSeadragon.Rect} rect
+     */
+    drawDebuggingRect(rect){
+        let context = this._outputContext;
+        context.save();
+        context.lineWidth = 2 * $.pixelDensityRatio;
+        context.strokeStyle = this.debugGridColor[0];
+        context.fillStyle = this.debugGridColor[0];
+
+        context.strokeRect(
+            rect.x * $.pixelDensityRatio,
+            rect.y * $.pixelDensityRatio,
+            rect.width * $.pixelDensityRatio,
+            rect.height * $.pixelDensityRatio
+        );
+
+        context.restore();
+    }
+    _getTextureDataFromTile(tile){
+        return tile.getCanvasContext().canvas;
+    }
+
+    // Private methods
+    _drawTile(tile, tiledImage, texture, viewMatrix, imageOpacity){
+
+        let gl = this._gl;
+
+        // x, y, w, h in viewport coords
+        let x = tile.positionedBounds.x;
+        let y = tile.positionedBounds.y;
+        let w = tile.positionedBounds.width;
+        let h = tile.positionedBounds.height;
+
+        let matrix = new Mat3([
+            w, 0, 0,
+            0, h, 0,
+            x, y, 1,
+        ]);
+
+
+        if(tile.flipped){
+            // flip the tile around the center of the unit quad
+            let t1 = Mat3.makeTranslation(0.5, 0);
+            let t2 = Mat3.makeTranslation(-0.5, 0);
+
+            // update the view matrix to account for this image's rotation
+            let localMatrix = t1.multiply(Mat3.makeScaling(-1, 1)).multiply(t2);
+            matrix = matrix.multiply(localMatrix);
+        }
+
+        let overallMatrix = viewMatrix.multiply(matrix);
+
+        if(tile.opacity !== 1 && tile.x === 0 && tile.y === 0){
+            // set opacity for this image
+            this._gl.uniform1f(this._glLocs.uOpacityMultiplier, imageOpacity * tile.opacity);
+        }
+
+        gl.uniformMatrix3fv(this._glLocs.uMatrix, false, overallMatrix.values);
+        gl.bindTexture(gl.TEXTURE_2D, texture);
+
+        if(this.continuousTileRefresh){
+            // Upload the image into the texture.
+            let tileContext = tile.getCanvasContext();
+            this._raiseTileDrawingEvent(tiledImage, this._outputContext, tile, tileContext);
+            this._uploadImageData(tileContext);
+        }
+
+
+        gl.drawArrays(gl.TRIANGLES, 0, 6);
+    }
+
+    _setupRenderer(){
+
+        if(!this._gl){
+            $.console.error('_setupCanvases must be called before _setupRenderer');
+        }
+
+        const vertexShaderProgram = `
+        attribute vec2 a_position;
+
+        uniform mat3 u_matrix;
+
+        varying vec2 v_texCoord;
+
+        void main() {
+        gl_Position = vec4(u_matrix * vec3(a_position, 1), 1);
+
+        // because we're using a unit quad we can just use
+        // the same data for our texcoords.
+        v_texCoord = a_position;
+        }
+        `;
+
+        const fragmentShaderProgram = `
+        precision mediump float;
+
+        // our texture
+        uniform sampler2D u_image;
+
+        // the texCoords passed in from the vertex shader.
+        varying vec2 v_texCoord;
+
+        // the opacity multiplier for the image
+        uniform float u_opacity_multiplier;
+
+        void main() {
+        gl_FragColor = texture2D(u_image, v_texCoord);
+        // gl_FragColor *= u_opacity_multiplier;
+        }
+        `;
+        let gl = this._gl;
+        this._glProgram = this.constructor.initShaderProgram(gl, vertexShaderProgram, fragmentShaderProgram);
+        gl.useProgram(this._glProgram);
+        gl.enable(gl.BLEND);
+        gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
+
+        this._glLocs = {
+            aPosition: gl.getAttribLocation(this._glProgram, 'a_position'),
+            uMatrix: gl.getUniformLocation(this._glProgram, 'u_matrix'),
+            uImage: gl.getUniformLocation(this._glProgram, 'u_image'),
+            uOpacityMultiplier: gl.getUniformLocation(this._glProgram, 'u_opacity_multiplier')
+        };
+
+        // provide texture coordinates for the rectangle.
+        this._glPositionBuffer = gl.createBuffer(); //keep reference to clear it later
+        gl.bindBuffer(gl.ARRAY_BUFFER, this._glPositionBuffer);
+        gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
+            0.0, 0.0,
+            1.0, 0.0,
+            0., 1.0,
+            0.0, 1.0,
+            1.0, 0.0,
+            1.0, 1.0]), gl.STATIC_DRAW);
+        gl.enableVertexAttribArray(this._glLocs.aPosition);
+        gl.vertexAttribPointer(this._glLocs.aPosition, 2, gl.FLOAT, false, 0, 0);
+
+    }
+
+    _setupCanvases(){
+        let _this = this;
+
+        this._outputCanvas = this.canvas; //output canvas
+        this._outputContext = this._outputCanvas.getContext('2d');
+
+        this._renderingCanvas = document.createElement('canvas');
+
+        this._clippingCanvas = document.createElement('canvas');
+        this._clippingContext = this._clippingCanvas.getContext('2d');
+        this._renderingCanvas.width = this._clippingCanvas.width = this._outputCanvas.width;
+        this._renderingCanvas.height = this._clippingCanvas.height = this._outputCanvas.height;
+
+        this._gl = this._renderingCanvas.getContext('webgl');
+
+        //make the additional canvas elements mirror size changes to the output canvas
+        this.viewer.addHandler("resize", function(){
+
+            if(_this._outputCanvas !== _this.viewer.drawer.canvas){
+                _this._outputCanvas.style.width = _this.viewer.drawer.canvas.clientWidth + 'px';
+                _this._outputCanvas.style.height = _this.viewer.drawer.canvas.clientHeight + 'px';
+            }
+
+            let viewportSize = _this._calculateCanvasSize();
+            if( _this._outputCanvas.width !== viewportSize.x ||
+                _this._outputCanvas.height !== viewportSize.y ) {
+                _this._outputCanvas.width = viewportSize.x;
+                _this._outputCanvas.height = viewportSize.y;
+            }
+
+            _this._renderingCanvas.style.width = _this._outputCanvas.clientWidth + 'px';
+            _this._renderingCanvas.style.height = _this._outputCanvas.clientHeight + 'px';
+            _this._renderingCanvas.width = _this._clippingCanvas.width = _this._outputCanvas.width;
+            _this._renderingCanvas.height = _this._clippingCanvas.height = _this._outputCanvas.height;
+            _this._gl.viewport(0, 0, _this._renderingCanvas.width, _this._renderingCanvas.height);
+        });
+    }
+
+
+    _tileReadyHandler(event){
+        let tile = event.tile;
+        let tileContext = tile.getCanvasContext();
+        let canvas = tileContext.canvas;
+        let texture = this._TextureMap.get(canvas);
+
+        // if this is a new image for us, create a texture
+        if(!texture){
+            let gl = this._gl;
+
+            // create a gl Texture for this tile and bind the canvas with the image data
+            texture = gl.createTexture();
+            // add it to our _TextureMap
+            this._TextureMap.set(canvas, texture);
+
+            gl.bindTexture(gl.TEXTURE_2D, texture);
+            // Set the parameters so we can render any size image.
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
+
+            // Upload the image into the texture.
+            this._uploadImageData(tileContext);
+
+        }
+
+    }
+
+    _uploadImageData(tileContext){
+        let gl = this._gl;
+        try{
+            let canvas = tileContext.canvas;
+            if(!canvas){
+                throw('Tile context does not have a canvas', tileContext);
+            }
+            gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, canvas);
+        } catch(e) {
+            $.console.error('Error uploading canvas data to webgl', e);
+        }
+    }
+
+    _imageUnloadedHandler(event){
+        let canvas = event.context2D.canvas;
+        this._cleanupImageData(canvas);
+    }
+
+    _cleanupImageData(tileCanvas){
+        let texture = this._TextureMap.get(tileCanvas);
+        //remove from the map
+        this._TextureMap.delete(tileCanvas);
+
+        //release the texture from the GPU
+        this._gl.deleteTexture(texture);
+    }
+    // private
+    // necessary for clip testing to pass (test uses spyOnce(drawer._setClip))
+    _setClip(rect){
+        this._clippingContext.beginPath();
+        this._clippingContext.rect(rect.x, rect.y, rect.width, rect.height);
+        this._clippingContext.clip();
+    }
+    _renderToClippingCanvas(item){
+        let _this = this;
+
+        this._clippingContext.clearRect(0, 0, this._clippingCanvas.width, this._clippingCanvas.height);
+        this._clippingContext.save();
+
+        if(item._clip){
+            var box = item.imageToViewportRectangle(item._clip, true);
+            var rect = this.viewportToDrawerRectangle(box);
+            this._setClip(rect);
+        }
+        if(item._croppingPolygons){
+            let polygons = item._croppingPolygons.map(function (polygon) {
+                return polygon.map(function (coord) {
+                    let point = item.imageToViewportCoordinates(coord.x, coord.y, true)
+                        .rotate(_this.viewer.viewport.getRotation(true), _this.viewer.viewport.getCenter(true));
+                    let clipPoint = _this._viewportCoordToDrawerCoord(point);
+                    return clipPoint;
+                });
+            });
+            this._clippingContext.beginPath();
+            polygons.forEach(function (polygon) {
+                polygon.forEach(function (coord, i) {
+                    _this._clippingContext[i === 0 ? 'moveTo' : 'lineTo'](coord.x, coord.y);
+                });
+            });
+            this._clippingContext.clip();
+        }
+
+        this._clippingContext.drawImage(this._renderingCanvas, 0, 0);
+
+        this._clippingContext.restore();
+    }
+
+    // private
+    _offsetForRotation(options) {
+        var point = options.point ?
+            options.point.times($.pixelDensityRatio) :
+            new $.Point(this._outputCanvas.width / 2, this._outputCanvas.height / 2);
+
+        var context = this._outputContext;
+        context.save();
+
+        context.translate(point.x, point.y);
+        if(this.viewport.flipped){
+          context.rotate(Math.PI / 180 * -options.degrees);
+          context.scale(-1, 1);
+        } else{
+          context.rotate(Math.PI / 180 * options.degrees);
+        }
+        context.translate(-point.x, -point.y);
+    }
+
+
+    /**
+     * @private
+     * @inner
+     * This function converts the given point from to the drawer coordinate by
+     * multiplying it with the pixel density.
+     * This function does not take rotation into account, thus assuming provided
+     * point is at 0 degree.
+     * @param {OpenSeadragon.Point} point - the pixel point to convert
+     * @returns {OpenSeadragon.Point} Point in drawer coordinate system.
+     */
+    _viewportCoordToDrawerCoord(point) {
+        var vpPoint = this.viewport.pixelFromPointNoRotate(point, true);
+        return new $.Point(
+            vpPoint.x * $.pixelDensityRatio,
+            vpPoint.y * $.pixelDensityRatio
+        );
+    }
+
+    // private
+    _drawDebugInfo( tilesToDraw, tiledImage, stroke, fill ) {
+
+        for ( var i = tilesToDraw.length - 1; i >= 0; i-- ) {
+            var tile = tilesToDraw[ i ].tile;
+            try {
+                this._drawDebugInfoOnTile(tile, tilesToDraw.length, i, tiledImage, stroke, fill);
+            } catch(e) {
+                $.console.error(e);
+            }
+        }
+    }
+    // private
+    _drawDebugInfoOnTile(tile, count, i, tiledImage, stroke, fill) {
+
+        var context = this._outputContext;
+        context.save();
+        context.lineWidth = 2 * $.pixelDensityRatio;
+        context.font = 'small-caps bold ' + (13 * $.pixelDensityRatio) + 'px arial';
+        context.strokeStyle = stroke;
+        context.fillStyle = fill;
+
+        if (this.viewport.getRotation(true) % 360 !== 0 ) {
+            this._offsetForRotation({degrees: this.viewport.getRotation(true)});
+        }
+        if (tiledImage.getRotation(true) % 360 !== 0) {
+            this._offsetForRotation({
+                degrees: tiledImage.getRotation(true),
+                point: tiledImage.viewport.pixelFromPointNoRotate(
+                    tiledImage._getRotationPoint(true), true)
+            });
+        }
+        if (tiledImage.viewport.getRotation(true) % 360 === 0 &&
+            tiledImage.getRotation(true) % 360 === 0) {
+            if(tiledImage._drawer.viewer.viewport.getFlip()) {
+                tiledImage._drawer._flip();
+            }
+        }
+
+        context.strokeRect(
+            tile.position.x * $.pixelDensityRatio,
+            tile.position.y * $.pixelDensityRatio,
+            tile.size.x * $.pixelDensityRatio,
+            tile.size.y * $.pixelDensityRatio
+        );
+
+        var tileCenterX = (tile.position.x + (tile.size.x / 2)) * $.pixelDensityRatio;
+        var tileCenterY = (tile.position.y + (tile.size.y / 2)) * $.pixelDensityRatio;
+
+        // Rotate the text the right way around.
+        context.translate( tileCenterX, tileCenterY );
+        context.rotate( Math.PI / 180 * -this.viewport.getRotation(true) );
+        context.translate( -tileCenterX, -tileCenterY );
+
+        if( tile.x === 0 && tile.y === 0 ){
+            context.fillText(
+                "Zoom: " + this.viewport.getZoom(),
+                tile.position.x * $.pixelDensityRatio,
+                (tile.position.y - 30) * $.pixelDensityRatio
+            );
+            context.fillText(
+                "Pan: " + this.viewport.getBounds().toString(),
+                tile.position.x * $.pixelDensityRatio,
+                (tile.position.y - 20) * $.pixelDensityRatio
+            );
+        }
+        context.fillText(
+            "Level: " + tile.level,
+            (tile.position.x + 10) * $.pixelDensityRatio,
+            (tile.position.y + 20) * $.pixelDensityRatio
+        );
+        context.fillText(
+            "Column: " + tile.x,
+            (tile.position.x + 10) * $.pixelDensityRatio,
+            (tile.position.y + 30) * $.pixelDensityRatio
+        );
+        context.fillText(
+            "Row: " + tile.y,
+            (tile.position.x + 10) * $.pixelDensityRatio,
+            (tile.position.y + 40) * $.pixelDensityRatio
+        );
+        context.fillText(
+            "Order: " + i + " of " + count,
+            (tile.position.x + 10) * $.pixelDensityRatio,
+            (tile.position.y + 50) * $.pixelDensityRatio
+        );
+        context.fillText(
+            "Size: " + tile.size.toString(),
+            (tile.position.x + 10) * $.pixelDensityRatio,
+            (tile.position.y + 60) * $.pixelDensityRatio
+        );
+        context.fillText(
+            "Position: " + tile.position.toString(),
+            (tile.position.x + 10) * $.pixelDensityRatio,
+            (tile.position.y + 70) * $.pixelDensityRatio
+        );
+
+        if (this.viewport.getRotation(true) % 360 !== 0 ) {
+            this._restoreRotationChanges();
+        }
+        if (tiledImage.getRotation(true) % 360 !== 0) {
+            this._restoreRotationChanges();
+        }
+
+        if (tiledImage.viewport.getRotation(true) % 360 === 0 &&
+            tiledImage.getRotation(true) % 360 === 0) {
+            if(tiledImage._drawer.viewer.viewport.getFlip()) {
+                tiledImage._drawer._flip();
+            }
+        }
+
+        context.restore();
+    }
+
+    // private
+    _restoreRotationChanges() {
+        var context = this._outputContext;
+        context.restore();
+    }
+
+    // modified from https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/Tutorial/Adding_2D_content_to_a_WebGL_context
+    static initShaderProgram(gl, vsSource, fsSource) {
+        const vertexShader = this.loadShader(gl, gl.VERTEX_SHADER, vsSource);
+        const fragmentShader = this.loadShader(gl, gl.FRAGMENT_SHADER, fsSource);
+
+        // Create the shader program
+
+        const shaderProgram = gl.createProgram();
+        gl.attachShader(shaderProgram, vertexShader);
+        gl.attachShader(shaderProgram, fragmentShader);
+        gl.linkProgram(shaderProgram);
+
+        // If creating the shader program failed, alert
+
+        if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
+        alert(
+            `Unable to initialize the shader program: ${gl.getProgramInfoLog(
+            shaderProgram
+            )}`
+        );
+        return null;
+        }
+
+        return shaderProgram;
+    }
+
+    //
+    // creates a shader of the given type, uploads the source and
+    // compiles it.
+    //
+    static loadShader(gl, type, source) {
+        const shader = gl.createShader(type);
+
+        // Send the source to the shader object
+
+        gl.shaderSource(shader, source);
+
+        // Compile the shader program
+
+        gl.compileShader(shader);
+
+        // See if it compiled successfully
+
+        if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
+        alert(
+            `An error occurred compiling the shaders: ${gl.getShaderInfoLog(shader)}`
+        );
+        gl.deleteShader(shader);
+        return null;
+        }
+
+        return shader;
+    }
+};
+
+
+}( OpenSeadragon ));
diff --git a/src/world.js b/src/world.js
index e766045d..30e9fc48 100644
--- a/src/world.js
+++ b/src/world.js
@@ -257,10 +257,10 @@ $.extend( $.World.prototype, $.EventSource.prototype, /** @lends OpenSeadragon.W
      */
     draw: function() {
         this.viewer.drawer.draw(this._items);
-        this._items.forEach(function(item){
-            item.setDrawn();
-        });
         this._needsDraw = false;
+        this._items.forEach(function(item){
+            this._needsDraw = item.setDrawn() || this._needsDraw || true;
+        });
     },
 
     /**
diff --git a/test/demo/threejsdrawer.js b/test/demo/threejsdrawer.js
index 73a257bf..5abcce17 100644
--- a/test/demo/threejsdrawer.js
+++ b/test/demo/threejsdrawer.js
@@ -1,4 +1,10 @@
 // import 'https://cdnjs.cloudflare.com/ajax/libs/three.js/0.149.0/three.min.js';
+
+// TO DO LIST:
+// TO DO: Viewport flip does not work right
+// TO DO: wrapHorizontal and wrapVertical do not work right with scaled TiledImages
+// TO DO: wrapping doesn't work right with resolution of wrapped part when zoomed in
+
 import '../lib/three.js';
 const THREE = window.THREE;
 
@@ -182,7 +188,7 @@ export class ThreeJSDrawer extends OpenSeadragon.DrawerBase{
 
         }
 
-        tiledImages.forEach(tiledImage => tiledImage._needsDraw = false);
+        //tiledImages.forEach(tiledImage => tiledImage.setDrawn());
     }
 
     // Public API required by all Drawer implementations
@@ -349,7 +355,7 @@ export class ThreeJSDrawer extends OpenSeadragon.DrawerBase{
     }
 
     _tileUnloadedHandler(event){
-        console.log('Tile unloaded',event);
+        // console.log('Tile unloaded',event);
         let tile = event.tile;
         if(!this._tileMap[tile.cacheKey]){
             //already cleaned up
@@ -581,6 +587,26 @@ export class ThreeJSDrawer extends OpenSeadragon.DrawerBase{
     }
 
     // private
+
+    /**
+     * @private
+     * @inner
+     * This function converts the given point from to the drawer coordinate by
+     * multiplying it with the pixel density.
+     * This function does not take rotation into account, thus assuming provided
+     * point is at 0 degree.
+     * @param {OpenSeadragon.Point} point - the pixel point to convert
+     * @returns {OpenSeadragon.Point} Point in drawer coordinate system.
+     */
+    _viewportCoordToDrawerCoord(point) {
+        let $ = OpenSeadragon;
+        var vpPoint = this.viewport.pixelFromPointNoRotate(point, true);
+        return new $.Point(
+            vpPoint.x * $.pixelDensityRatio,
+            vpPoint.y * $.pixelDensityRatio
+        );
+    }
+
     _offsetForRotation(options) {
         var point = options.point ?
             options.point.times(OpenSeadragon.pixelDensityRatio) :
diff --git a/test/demo/webgl.html b/test/demo/webgl.html
index 196a65bc..bbee069c 100644
--- a/test/demo/webgl.html
+++ b/test/demo/webgl.html
@@ -68,7 +68,28 @@
 </head>
 <body>
     <div class="content">
-        <h2>Use a WebGL drawer implementation (using three.js) instead of the default context2d drawer</h2>
+
+        <h2>Compare behavior of <strong>Context2d</strong> and <strong>WebGL</strong> (via three.js) drawers</h2>
+        <div class="mirrored">
+            <div>
+                <h3>Context2d drawer (default in OSD &lt;= 4.1.0)</h3>
+                <div id="context2d" class="viewer-container"></div>
+            </div>
+
+            <div>
+                <h3>New WebGL drawer</h3>
+                <div id="webgl" class="viewer-container"></div>
+            </div>
+        </div>
+
+
+        <div id="image-picker">
+            <h3>Image options (drag and drop to re-order images)</h3>
+
+
+        </div>
+
+        <h2>Use a custom plugin drawer - example using three.js</h2>
         <div class="mirrored">
             <div>
                 <div class="description">
@@ -97,74 +118,6 @@
         </div>
 
 
-        <h2>Compare behavior of <strong>Context2d</strong> and <strong>WebGL</strong> (via three.js) drawers</h2>
-        <div class="mirrored">
-            <div>
-                <h3>Use default OpenSeadragon viewer to pan/zoom</h3>
-                <div id="contentDiv" class="viewer-container"></div>
-            </div>
-
-            <div>
-                <h3>WebGL drawer linked using event listeners </h3>
-                <div id="three-canvas-container" class="viewer-container"></div>
-            </div>
-        </div>
-
-
-        <div id="image-picker">
-            <h3>Image options (drag and drop to re-order images)</h3>
-            <!-- <div class="image-options">
-                <span class="ui-icon ui-icon-arrowthick-2-n-s"></span>
-                <label><input type="checkbox" checked data-image="rainbow" class="toggle"> Rainbow Grid</label>
-                <div class="option-grid">
-                    <label>X: <input type="number" value="0" data-image="rainbow" data-field="x"> </label>
-                    <label>Y: <input type="number" value="0" data-image="rainbow" data-field="y"> </label>
-                    <label>Width: <input type="number" value="1" data-image="rainbow" data-field="width" min="0"> </label>
-                    <label>Degrees: <input type="number" value="0" data-image="rainbow" data-field="degrees"> </label>
-                    <label>Opacity: <input type="number" value="1" data-image="rainbow" data-field="opacity" min="0" max="1" step="0.2"> </label>
-                    <label>Flipped: <input type="checkbox" data-image="rainbow" data-field="flipped"></label>
-                    <label>Cropped: <input type="checkbox" data-image="rainbow" data-field="cropped"></label>
-                    <label>Debug: <input type="checkbox" data-image="rainbow" data-field="debug"></label>
-                    <label>Composite: <select data-image="rainbow" data-field="composite"></select></label>
-                    <label>Wrapping: <select data-image="rainbow" data-field="wrapping"></select></label>
-                </div>
-
-            </div>
-            <div class="image-options">
-                <span class="ui-icon ui-icon-arrowthick-2-n-s"></span>
-                <label><input type="checkbox" data-image="leaves" class="toggle"> Leaves</label>
-                <div class="option-grid">
-                    <label>X: <input type="number" value="0" data-image="leaves" data-field="x"> </label>
-                    <label>Y: <input type="number" value="0" data-image="leaves" data-field="y"> </label>
-                    <label>Width: <input type="number" value="1" data-image="leaves" data-field="width" min="0"> </label>
-                    <label>Degrees: <input type="number" value="0" data-image="leaves" data-field="degrees"> </label>
-                    <label>Opacity: <input type="number" value="1" data-image="leaves" data-field="opacity" min="0" max="1" step="0.2"> </label>
-                    <label>Flipped: <input type="checkbox" data-image="leaves" data-field="flipped"></label>
-                    <label>Cropped: <input type="checkbox" data-image="leaves" data-field="cropped"></label>
-                    <label>Debug: <input type="checkbox" data-image="leaves" data-field="debug"></label>
-                    <label>Composite: <select data-image="leaves" data-field="composite" ></select></label>
-                    <label>Wrapping: <select data-image="leaves" data-field="wrapping"></select></label>
-                </div>
-            </div>
-            <div class="image-options">
-                <span class="ui-icon ui-icon-arrowthick-2-n-s"></span>
-                <label><input type="checkbox" data-image="bblue" class="toggle"> BBlue PNG</label>
-                <div class="option-grid">
-                    <label>X: <input type="number" value="0" data-image="bblue" data-field="x"> </label>
-                    <label>Y: <input type="number" value="0" data-image="bblue" data-field="y"> </label>
-                    <label>Width: <input type="number" value="1" data-image="bblue" data-field="width" min="0"> </label>
-                    <label>Degrees: <input type="number" value="0" data-image="bblue" data-field="degrees"> </label>
-                    <label>Opacity: <input type="number" value="1" data-image="bblue" data-field="opacity" min="0" max="1" step="0.2"> </label>
-                    <label>Flipped: <input type="checkbox" data-image="bblue" data-field="flipped"></label>
-                    <label>Cropped: <input type="checkbox" data-image="bblue" data-field="cropped"></label>
-                    <label>Debug: <input type="checkbox" data-image="bblue" data-field="debug"></label>
-                    <label>Composite: <select data-image="bblue" data-field="composite"></select></label>
-                    <label>Wrapping: <select data-image="bblue" data-field="wrapping"></select></label>
-                </div>
-            </div> -->
-
-        </div>
-
 
         <h2>HTMLDrawer: legacy pre-HTML5 drawer that uses &lt;img&gt; elements for tiles</h2>
         <div class="mirrored">
diff --git a/test/demo/webgl.js b/test/demo/webgl.js
index cdcd71e5..471d750d 100644
--- a/test/demo/webgl.js
+++ b/test/demo/webgl.js
@@ -1,8 +1,3 @@
-//imports
-import { ThreeJSDrawer } from './threejsdrawer.js';
-// import { default as Stats } from "https://cdnjs.cloudflare.com/ajax/libs/stats.js/17/Stats.js";
-//globals
-// const canvas = document.querySelector('#three-canvas');
 const sources = {
     "rainbow":"../data/testpattern.dzi",
     "leaves":"../data/iiif_2_0_sizes/info.json",
@@ -25,63 +20,102 @@ var stats = null;
 // document.body.appendChild( stats.dom );
 
 
-//Double viewer setup for comparison - Context2dDrawer and ThreeJSDrawer
+//Double viewer setup for comparison - Context2dDrawer and WebGLDrawer
 
-var viewer = window.viewer = OpenSeadragon({
-    id: "contentDiv",
+let viewer1 = window.viewer1 = OpenSeadragon({
+    id: "context2d",
     prefixUrl: "../../build/openseadragon/images/",
-    // minZoomImageRatio:0.8,
-    // maxZoomPixelRatio:0.5,
     minZoomImageRatio:0.01,
     maxZoomPixelRatio:100,
     smoothTileEdgesMinZoom:1.1,
     crossOriginPolicy: 'Anonymous',
     ajaxWithCredentials: false,
-    drawer:'context2d',
+    // maxImageCacheCount: 30,
+    drawer:'webgl',
+    blendTime:0
 });
 
-
-// Mirror the interactive viewer with Context2dDrawer onto a separate canvas using ThreeJSDrawer
-let threeRenderer = window.threeRenderer = new ThreeJSDrawer({viewer, viewport: viewer.viewport, element:viewer.element, stats: stats});
-//make the test canvas mirror all changes to the viewer canvas
-let viewerCanvas = viewer.drawer.canvas;
-let canvas = threeRenderer.canvas;
-let canvasContainer = $('#three-canvas-container').append(canvas);
-viewer.addHandler("resize", function(){
-    canvasContainer[0].style.width = viewerCanvas.clientWidth+'px';
-    canvasContainer[0].style.height = viewerCanvas.clientHeight+'px';
-    // canvas.width = viewerCanvas.width;
-    // canvas.height = viewerCanvas.height;
-});
-
-
-// Single viewer showing how to use plugin Drawer via configuration
-// Also shows sequence mode
-var viewer2 = window.viewer2 = OpenSeadragon({
-    id: "three-viewer",
+let viewer2 = window.viewer2 = OpenSeadragon({
+    id: "webgl",
     prefixUrl: "../../build/openseadragon/images/",
     minZoomImageRatio:0.01,
-    drawer: ThreeJSDrawer,
-    tileSources: [sources['leaves'], sources['rainbow'], sources['duomo']],
-    sequenceMode: true,
-    imageSmoothingEnabled: false,
+    maxZoomPixelRatio:100,
+    smoothTileEdgesMinZoom:1.1,
     crossOriginPolicy: 'Anonymous',
-    ajaxWithCredentials: false
+    ajaxWithCredentials: false,
+    // maxImageCacheCount: 30,
+    drawer:'webgl',
+    blendTime:0.0
 });
 
-// Single viewer showing how to use plugin Drawer via configuration
-// Also shows sequence mode
-var viewer3 = window.viewer3 = OpenSeadragon({
-    id: "htmldrawer",
-    drawer:'html',
-    prefixUrl: "../../build/openseadragon/images/",
-    minZoomImageRatio:0.01,
-    customDrawer: OpenSeadragon.HTMLDrawer,
-    tileSources: [sources['leaves'], sources['rainbow'], sources['duomo']],
-    sequenceMode: true,
-    crossOriginPolicy: 'Anonymous',
-    ajaxWithCredentials: false
-});
+// Sync navigation of viewer1 and viewer 2
+var viewer1Leading = false;
+var viewer2Leading = false;
+
+var viewer1Handler = function() {
+    if (viewer2Leading) {
+        return;
+    }
+
+    viewer1Leading = true;
+    viewer2.viewport.zoomTo(viewer1.viewport.getZoom());
+    viewer2.viewport.panTo(viewer1.viewport.getCenter());
+    viewer2.viewport.rotateTo(viewer1.viewport.getRotation());
+    viewer2.viewport.setFlip(viewer1.viewport.flipped);
+    viewer1Leading = false;
+};
+
+var viewer2Handler = function() {
+    if (viewer1Leading) {
+        return;
+    }
+
+    viewer2Leading = true;
+    viewer1.viewport.zoomTo(viewer2.viewport.getZoom());
+    viewer1.viewport.panTo(viewer2.viewport.getCenter());
+    viewer1.viewport.rotateTo(viewer2.viewport.getRotation());
+    viewer1.viewport.setFlip(viewer1.viewport.flipped);
+    viewer2Leading = false;
+};
+
+viewer1.addHandler('zoom', viewer1Handler);
+viewer2.addHandler('zoom', viewer2Handler);
+viewer1.addHandler('pan', viewer1Handler);
+viewer2.addHandler('pan', viewer2Handler);
+viewer1.addHandler('rotate', viewer1Handler);
+viewer2.addHandler('rotate', viewer2Handler);
+viewer1.addHandler('flip', viewer1Handler);
+viewer2.addHandler('flip', viewer2Handler);
+
+
+// // Single viewer showing how to use plugin Drawer via configuration
+// // Also shows sequence mode
+// var viewer3 = window.viewer3 = OpenSeadragon({
+//     id: "three-viewer",
+//     prefixUrl: "../../build/openseadragon/images/",
+//     minZoomImageRatio:0.01,
+//     drawer: ThreeJSDrawer,
+//     tileSources: [sources['leaves'], sources['rainbow'], sources['duomo']],
+//     sequenceMode: true,
+//     imageSmoothingEnabled: false,
+//     crossOriginPolicy: 'Anonymous',
+//     ajaxWithCredentials: false
+// });
+
+// // Single viewer showing how to use plugin Drawer via configuration
+// // Also shows sequence mode
+// var viewer4 = window.viewer4 = OpenSeadragon({
+//     id: "htmldrawer",
+//     drawer:'html',
+//     blendTime:2,
+//     prefixUrl: "../../build/openseadragon/images/",
+//     minZoomImageRatio:0.01,
+//     customDrawer: OpenSeadragon.HTMLDrawer,
+//     tileSources: [sources['leaves'], sources['rainbow'], sources['duomo']],
+//     sequenceMode: true,
+//     crossOriginPolicy: 'Anonymous',
+//     ajaxWithCredentials: false
+// });
 
 
 
@@ -90,11 +124,18 @@ $('#three-viewer').resizable(true);
 $('#contentDiv').resizable(true);
 $('#image-picker').sortable({
     update: function(event, ui){
-        let thisItem = ui.item.find('.toggle').data('item');
-        let items = $('#image-picker input.toggle:checked').toArray().map(item=>$(item).data('item'));
+        let thisItem = ui.item.find('.toggle').data('item1');
+        let items = $('#image-picker input.toggle:checked').toArray().map(item=>$(item).data('item1'));
         let newIndex = items.indexOf(thisItem);
         if(thisItem){
-            viewer.world.setItemIndex(thisItem, newIndex);
+            viewer1.world.setItemIndex(thisItem, newIndex);
+        }
+
+        thisItem = ui.item.find('.toggle').data('item2');
+        items = $('#image-picker input.toggle:checked').toArray().map(item=>$(item).data('item2'));
+        newIndex = items.indexOf(thisItem);
+        if(thisItem){
+            viewer2.world.setItemIndex(thisItem, newIndex);
         }
     }
 });
@@ -110,12 +151,13 @@ Object.keys(sources).forEach((key, index)=>{
 $('#image-picker input.toggle').on('change',function(){
     let data = $(this).data();
     if(this.checked){
-        addTileSource(data.image, this);
-
+        addTileSource(viewer1, data.image, this);
+        // addTileSource(viewer2, data.image, this);
     } else {
-        if(data.item){
-            viewer.world.removeItem(data.item);
-            $(this).data('item',null);
+        if(data.item1){
+            viewer1.world.removeItem(data.item1);
+            // viewer2.world.removeItem(data.item2);
+            $(this).data({item1: null, item2: null});
         }
     }
 }).trigger('change');
@@ -123,7 +165,13 @@ $('#image-picker input.toggle').on('change',function(){
 $('#image-picker input:not(.toggle)').on('change',function(){
     let data = $(this).data();
     let value = $(this).val();
-    let tiledImage = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item');
+    let tiledImage1 = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item1');
+    let tiledImage2 = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item2');
+    updateTiledImage(tiledImage1, data, value, this);
+    updateTiledImage(tiledImage2, data, value, this);
+});
+
+function updateTiledImage(tiledImage, data, value, item){
     if(tiledImage){
         //item = tiledImage
         let field = data.field;
@@ -142,16 +190,16 @@ $('#image-picker input:not(.toggle)').on('change',function(){
         } else if (field == 'opacity'){
             tiledImage.setOpacity(Number(value));
         } else if (field == 'flipped'){
-            tiledImage.setFlip($(this).prop('checked'));
+            tiledImage.setFlip($(item).prop('checked'));
         } else if (field == 'cropped'){
-            if( $(this).prop('checked') ){
+            if( $(item).prop('checked') ){
                 let croppingPolygons = [ [{x:200, y:200}, {x:800, y:200}, {x:500, y:800}] ];
                 tiledImage.setCroppingPolygons(croppingPolygons);
             } else {
                 tiledImage.resetCroppingPolygons();
             }
         } else if (field == 'clipped'){
-            if( $(this).prop('checked') ){
+            if( $(item).prop('checked') ){
                 let clipRect = new OpenSeadragon.Rect(2000, 0, 3000, 4000);
                 tiledImage.setClip(clipRect);
             } else {
@@ -159,26 +207,40 @@ $('#image-picker input:not(.toggle)').on('change',function(){
             }
         }
         else if (field == 'debug'){
-            if( $(this).prop('checked') ){
+            if( $(item).prop('checked') ){
                 tiledImage.debugMode = true;
             } else {
                 tiledImage.debugMode = false;
             }
         }
     }
-});
+}
 
 $('.image-options select[data-field=composite]').append(getCompositeOperationOptions()).on('change',function(){
     let data = $(this).data();
-    let tiledImage = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item');
-    if(tiledImage){
-        tiledImage.setCompositeOperation(this.value == 'null' ? null : this.value);
+    let tiledImage1 = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item1');
+    if(tiledImage1){
+        tiledImage1.setCompositeOperation(this.value == 'null' ? null : this.value);
+    }
+    let tiledImage2 = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item2');
+    if(tiledImage2){
+        tiledImage2.setCompositeOperation(this.value == 'null' ? null : this.value);
     }
 }).trigger('change');
 
 $('.image-options select[data-field=wrapping]').append(getWrappingOptions()).on('change',function(){
     let data = $(this).data();
-    let tiledImage = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item');
+    let tiledImage = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item1');
+    if(tiledImage){
+        switch(this.value){
+            case "None": tiledImage.wrapHorizontal = tiledImage.wrapVertical = false; break;
+            case "Horizontal": tiledImage.wrapHorizontal = true; tiledImage.wrapVertical = false; break;
+            case "Vertical": tiledImage.wrapHorizontal = false; tiledImage.wrapVertical = true; break;
+            case "Both": tiledImage.wrapHorizontal = tiledImage.wrapVertical = true; break;
+        }
+        tiledImage.viewer.raiseEvent('opacity-change');//trigger a redraw for the webgl renderer. TODO: fix this hack.
+    }
+    tiledImage = $(`#image-picker input.toggle[data-image=${data.image}]`).data('item2');
     if(tiledImage){
         switch(this.value){
             case "None": tiledImage.wrapHorizontal = tiledImage.wrapVertical = false; break;
@@ -220,7 +282,7 @@ function getCompositeOperationOptions(){
 
 }
 
-function addTileSource(image, checkbox){
+function addTileSource(viewer, image, checkbox){
     let options = $(`#image-picker input[data-image=${image}][type=number]`).toArray().reduce((acc, input)=>{
         let field = $(input).data('field');
         if(field){
@@ -236,10 +298,11 @@ function addTileSource(image, checkbox){
 
     let tileSource = sources[image];
     if(tileSource){
-        viewer.addTiledImage({tileSource: tileSource, ...options, index: insertionIndex});
-        viewer.world.addOnceHandler('add-item',function(ev){
+        viewer&&viewer.addTiledImage({tileSource: tileSource, ...options, index: insertionIndex});
+        viewer&&viewer.world.addOnceHandler('add-item',function(ev){
             let item = ev.item;
-            $(checkbox).data('item',item);
+            let field = viewer === viewer1 ? 'item1' : 'item2';
+            $(checkbox).data(field,item);
             item.source.hasTransparency = ()=>true; //simulate image with transparency, to show seams in default renderer
         });
     }
diff --git a/test/modules/ajax-tiles.js b/test/modules/ajax-tiles.js
index 39a37876..68e5ab16 100644
--- a/test/modules/ajax-tiles.js
+++ b/test/modules/ajax-tiles.js
@@ -56,6 +56,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
diff --git a/test/modules/basic.js b/test/modules/basic.js
index 49c70365..1315d96d 100644
--- a/test/modules/basic.js
+++ b/test/modules/basic.js
@@ -19,6 +19,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
@@ -319,8 +322,8 @@
                     height: 155
                 } ]
         } );
-        viewer.addOnceHandler('tile-drawn', function() {
-            assert.ok(OpenSeadragon.isCanvasTainted(viewer.drawer.context.canvas),
+        viewer.addOnceHandler('tile-drawn', function(event) {
+            assert.ok(OpenSeadragon.isCanvasTainted(event.tile.getCanvasContext().canvas),
                 "Canvas should be tainted.");
             done();
         });
@@ -339,8 +342,8 @@
                     height: 155
                 } ]
         } );
-        viewer.addOnceHandler('tile-drawn', function() {
-            assert.ok(!OpenSeadragon.isCanvasTainted(viewer.drawer.context.canvas),
+        viewer.addOnceHandler('tile-drawn', function(event) {
+            assert.ok(!OpenSeadragon.isCanvasTainted(event.tile.getCanvasContext().canvas),
                 "Canvas should not be tainted.");
             done();
         });
@@ -363,8 +366,8 @@
             },
             crossOriginPolicy : false
         } );
-        viewer.addOnceHandler('tile-drawn', function() {
-            assert.ok(OpenSeadragon.isCanvasTainted(viewer.drawer.context.canvas),
+        viewer.addOnceHandler('tile-drawn', function(event) {
+            assert.ok(OpenSeadragon.isCanvasTainted(event.tile.getCanvasContext().canvas),
                 "Canvas should be tainted.");
             done();
         });
@@ -387,8 +390,8 @@
                 crossOriginPolicy : "Anonymous"
             }
         } );
-        viewer.addOnceHandler('tile-drawn', function() {
-            assert.ok(!OpenSeadragon.isCanvasTainted(viewer.drawer.context.canvas),
+        viewer.addOnceHandler('tile-drawn', function(event) {
+            assert.ok(!OpenSeadragon.isCanvasTainted(event.tile.getCanvasContext().canvas),
                 "Canvas should not be tainted.");
             done();
         });
diff --git a/test/modules/controls.js b/test/modules/controls.js
index 7e774d9b..0bf68ea3 100644
--- a/test/modules/controls.js
+++ b/test/modules/controls.js
@@ -19,6 +19,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
diff --git a/test/modules/drawer.js b/test/modules/drawer.js
index d67df286..e4f507a3 100644
--- a/test/modules/drawer.js
+++ b/test/modules/drawer.js
@@ -13,7 +13,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
-
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
             viewer = null;
         }
     });
@@ -42,7 +44,8 @@
     QUnit.test('rotation', function(assert) {
         var done = assert.async();
         createViewer({
-            tileSources: '/test/data/testpattern.dzi'
+            tileSources: '/test/data/testpattern.dzi',
+            drawer: 'context2d', // this test only makes sense for certain drawers
         });
 
         viewer.addHandler('open', function handler(event) {
@@ -62,8 +65,8 @@
             debugMode: true
         });
 
-        Util.spyOnce(viewer.drawer, 'drawDebugInfo', function() {
-            assert.ok(true, 'drawDebugInfo is called');
+        Util.spyOnce(viewer.drawer, '_drawDebugInfo', function() {
+            assert.ok(true, '_drawDebugInfo is called');
             done();
         });
     });
@@ -72,7 +75,8 @@
     QUnit.test('sketchCanvas', function(assert) {
         var done = assert.async();
         createViewer({
-            tileSources: '/test/data/testpattern.dzi'
+            tileSources: '/test/data/testpattern.dzi',
+            drawer: 'context2d' // test only makes sense for this drawer
         });
         var drawer = viewer.drawer;
 
diff --git a/test/modules/events.js b/test/modules/events.js
index 010b08b9..698a3ef2 100644
--- a/test/modules/events.js
+++ b/test/modules/events.js
@@ -20,6 +20,9 @@
             if ( viewer && viewer.close ) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
@@ -1155,6 +1158,7 @@
     // ----------
     QUnit.test( 'Viewer: event count test with \'tile-drawing\'', function (assert) {
         var done = assert.async();
+        var previousValue = viewer.drawer.continuousTileRefresh;
         assert.ok(viewer.numberOfHandlers('tile-drawing') === 0,
             "'tile-drawing' event is empty by default.");
 
@@ -1162,6 +1166,7 @@
             viewer.removeHandler( 'tile-drawing', tileDrawing );
             assert.ok(viewer.numberOfHandlers('tile-drawing') === 0,
                 "'tile-drawing' deleted: count is 0.");
+            viewer.drawer.continuousTileRefresh = previousValue; // reset property
             viewer.close();
             done();
         };
@@ -1180,11 +1185,14 @@
         assert.ok(viewer.numberOfHandlers('tile-drawing') === 1,
             "'tile-drawing' deleted once: count is 1.");
 
+        viewer.drawer.continuousTileRefresh = true; // set to true so the tile-drawing event fires
         viewer.open( '/test/data/testpattern.dzi' );
     } );
 
     QUnit.test( 'Viewer: tile-drawing event', function (assert) {
         var done = assert.async();
+        var previousValue = viewer.drawer.continuousTileRefresh;
+
         var tileDrawing = function ( event ) {
             viewer.removeHandler( 'tile-drawing', tileDrawing );
             assert.ok( event, 'Event handler should be invoked' );
@@ -1194,10 +1202,12 @@
                 assert.ok(event.tile, "Tile should be set");
                 assert.ok(event.rendered, "Rendered should be set");
             }
+            viewer.drawer.continuousTileRefresh = previousValue; // reset property
             viewer.close();
             done();
         };
 
+        viewer.drawer.continuousTileRefresh = true; // set to true so the tile-drawing event fires
         viewer.addHandler( 'tile-drawing', tileDrawing );
         viewer.open( '/test/data/testpattern.dzi' );
     } );
diff --git a/test/modules/formats.js b/test/modules/formats.js
index 867bcd17..7a30dd84 100644
--- a/test/modules/formats.js
+++ b/test/modules/formats.js
@@ -5,15 +5,26 @@
     // This module tests whether our various file formats can be opened.
     // TODO: Add more file formats (with corresponding test data).
 
+    var viewer = null;
+
     QUnit.module('Formats', {
         beforeEach: function () {
             var example = document.createElement("div");
             example.id = "example";
             document.getElementById("qunit-fixture").appendChild(example);
+        },
+        afterEach: function () {
+            if ( viewer && viewer.close ) {
+                viewer.close();
+            }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
+
+            viewer = null;
         }
     });
 
-    var viewer = null;
 
     // ----------
     var testOpenUrl = function(relativeUrl, assert) {
diff --git a/test/modules/imageloader.js b/test/modules/imageloader.js
index 1e201b10..12842dfd 100644
--- a/test/modules/imageloader.js
+++ b/test/modules/imageloader.js
@@ -18,6 +18,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
diff --git a/test/modules/multi-image.js b/test/modules/multi-image.js
index bc6c6908..9b22b8cf 100644
--- a/test/modules/multi-image.js
+++ b/test/modules/multi-image.js
@@ -19,6 +19,9 @@
             if ( viewer && viewer.close ) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
             $("#example").remove();
diff --git a/test/modules/navigator.js b/test/modules/navigator.js
index 0859abb9..1209b3f8 100644
--- a/test/modules/navigator.js
+++ b/test/modules/navigator.js
@@ -41,6 +41,15 @@
             }
 
             resetTestVariables();
+
+            if ( viewer && viewer.close ) {
+                viewer.close();
+            }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
+
+            viewer = null;
         }
     });
 
diff --git a/test/modules/overlays.js b/test/modules/overlays.js
index 3a7a8877..bafc0d35 100644
--- a/test/modules/overlays.js
+++ b/test/modules/overlays.js
@@ -16,6 +16,14 @@
         },
         afterEach: function() {
             resetTestVariables();
+            if ( viewer && viewer.close ) {
+                viewer.close();
+            }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
+
+            viewer = null;
         }
     });
 
diff --git a/test/modules/referencestrip.js b/test/modules/referencestrip.js
index 2514dbd2..25ae1b9b 100644
--- a/test/modules/referencestrip.js
+++ b/test/modules/referencestrip.js
@@ -13,6 +13,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
diff --git a/test/modules/tiledimage.js b/test/modules/tiledimage.js
index a580e188..6c7cef56 100644
--- a/test/modules/tiledimage.js
+++ b/test/modules/tiledimage.js
@@ -20,6 +20,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
@@ -132,6 +135,8 @@
     QUnit.test('update', function(assert) {
         var done = assert.async();
         var handlerCount = 0;
+        var testTileDrawingEvent = viewer.drawerOptions.type === 'context2d';
+        let expectedHandlers = testTileDrawingEvent ? 4 : 3;
 
         viewer.addHandler('open', function(event) {
             var image = viewer.world.getItemAt(0);
@@ -160,15 +165,18 @@
                 assert.ok(event.tile, 'update-tile event includes tile');
             });
 
-            viewer.addHandler('tile-drawing', function tileDrawingHandler(event) {
-                viewer.removeHandler('tile-drawing', tileDrawingHandler);
-                handlerCount++;
-                assert.equal(event.eventSource, viewer, 'sender of tile-drawing event was viewer');
-                assert.equal(event.tiledImage, image, 'tiledImage of update-level event is correct');
-                assert.ok(event.tile, 'tile-drawing event includes a tile');
-                assert.ok(event.context, 'tile-drawing event includes a context');
-                assert.ok(event.rendered, 'tile-drawing event includes a rendered');
-            });
+            if(testTileDrawingEvent){
+                viewer.addHandler('tile-drawing', function tileDrawingHandler(event) {
+                    viewer.removeHandler('tile-drawing', tileDrawingHandler);
+                    handlerCount++;
+                    assert.equal(event.eventSource, viewer, 'sender of tile-drawing event was viewer');
+                    assert.equal(event.tiledImage, image, 'tiledImage of update-level event is correct');
+                    assert.ok(event.tile, 'tile-drawing event includes a tile');
+                    assert.ok(event.context, 'tile-drawing event includes a context');
+                    assert.ok(event.rendered, 'tile-drawing event includes a rendered');
+                });
+            }
+
 
             viewer.addHandler('tile-drawn', function tileDrawnHandler(event) {
                 viewer.removeHandler('tile-drawn', tileDrawnHandler);
@@ -177,11 +185,10 @@
                 assert.equal(event.tiledImage, image, 'tiledImage of update-level event is correct');
                 assert.ok(event.tile, 'tile-drawn event includes tile');
 
-                assert.equal(handlerCount, 4, 'correct number of handlers called');
+                assert.equal(handlerCount, expectedHandlers, 'correct number of handlers called');
                 done();
             });
 
-            //image.draw(); // TO DO: Is this necessary for the test? It will now fail since tiledImage.draw() is not a thing.
             viewer.drawer.draw( [ image ] );
         });
 
diff --git a/test/modules/tilesource-dynamic-url.js b/test/modules/tilesource-dynamic-url.js
index d7678805..f07fedc0 100644
--- a/test/modules/tilesource-dynamic-url.js
+++ b/test/modules/tilesource-dynamic-url.js
@@ -132,6 +132,10 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
+
             viewer = null;
 
             OpenSeadragon.makeAjaxRequest = OriginalAjax;
diff --git a/test/modules/units.js b/test/modules/units.js
index ee47ebe1..160cea35 100644
--- a/test/modules/units.js
+++ b/test/modules/units.js
@@ -22,6 +22,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
diff --git a/test/modules/viewport.js b/test/modules/viewport.js
index 6362a296..c805945e 100644
--- a/test/modules/viewport.js
+++ b/test/modules/viewport.js
@@ -24,6 +24,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }
diff --git a/test/modules/world.js b/test/modules/world.js
index 9c705345..29830f7f 100644
--- a/test/modules/world.js
+++ b/test/modules/world.js
@@ -19,6 +19,9 @@
             if (viewer && viewer.close) {
                 viewer.close();
             }
+            if (viewer && viewer.destroy){
+                viewer.destroy();
+            }
 
             viewer = null;
         }