For the dropdown search on single select instances, the search
focus should happen in a `setTimeout` call. This is because many
browsers trigger the element focus on `click`, so our original call
to `$search.focus` will essentially be ignored. By also focusing
it in a timeout, this will delay the call to `focus` so the browser
will no longer take it back.
We do not have this issue for multiple select instances, as they
trigger the `open` event on `click`, instead of using `mousedown`.
This fixes https://github.com/select2/select2/issues/2900.
68286e3cff