* Adjust margin for sticky elements on modal

Previously white space was visible to the right of sticky elements due to right padding being added to the body. This fixes #23661.

* Add unit tests for margin of sticky elements on modal

ESC can be used to close modals, but on OS X/macOS this also jumps out
of full-screen mode. `preventDefault` suppresses this.

Fix Modal documentation about _handleUpdate method + move to public scope

* Replace element.offet* by getBoundingClientRect()

* Use variable to store BoundingClientRect

* Fix cc issue...

* Throw error when a plugin is in transition
* Add unit tests about plugins in transition

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Mostly KeyboardEvent.which and MouseEvent.which values.

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Fixes #19878

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Modal: Ignore spurious `focus` event that Firefox fires at `document` when switching back to its tab

Fixes #18365
Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1228802

Errata from:
* 8ce5da30
* 5d3144e8
which missed a few numbers.

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We want to sum two numbers, not concatenate their stringifications.
Closes #17457 by merging a tweaked version of it.