This fixes an issue where most keys would not trigger the search
in Firefox for Android. There were only a few keys which would
trigger `keyup`, such as space and enter (the search icon), but
they were not consistent. We know that they were being triggered
though, as you could type "new " (note the space) and it would
trigger a search, giving us all states that started with "new".
The problem is that Firefox for Android does not consistently
trigger `keyup` and `keydown` events when a keyboard is used. To
work around the issue, we are now using the `input` event in
replacement of the `keyup` event, which was used to trigger the
search. While this is not an actual `KeyboardEvent` in Chrome, and
lacks some of the important metadata such as `which`/`key` in
Firefox, it works for our implementation.
As the `input` event is not supported in older browsers, such as
those before Internet Explorer 9, we have to listen for both the
old `keyup` event and the new `input` event. As the `input` event
is always triggered before the `keyup` event, we unbind the `keyup`
event automatically to prevent searches from being triggered twice.
This solution was discovered in a blog post by Mathias Bynens at
https://mathiasbynens.be/notes/oninput.
**Note:** The ability to backspace in a blank text field on multiple
selects in order to remove the last selected item does not work in
Firefox for Android because the `keydown` event does not trigger
when the text field is empty. Users can still use the "x" icon
provided at the start of every selected option to achieve the same
effect.
This closes https://github.com/select2/select2/issues/2997.
c0839b4f93