Bug 1909020 - Make createDragEventObject() in EventUtils.js aware of HiDPI environments r=smaug
`synthesizePlainDragAndDrop()` sends `drop` event [1] via `PresShell` [2].
Then, the `screenX` and `screenY` values are set to
`WidgetEvent::mRefPoint` [3]. Then, the `drop` event's position is
recorded before dispatch [4]. So, `drop` event is fired on the target
but the `mRefPoint` may be outside the target. Finally, synthesized
`eMouseMove` after `eDrop` will be fired on the wrong element which is
different from `eDrop`'s target. This caused the failures of
`test_synthmousemove_after_dnd.html` and `test_dragdrop.html` on Android.
Perhaps, we should improve `nsDOMWindowUtils` or something lower layer
later. Instead, this patch fixes in `EventUtils.js` level. This makes
the `createDragEventObject()`.
1. https://searchfox.org/mozilla-central/rev/6a72a6d20eeb1b20b93862a79166938d6ce794a0/testing/mochitest/tests/SimpleTest/EventUtils.js#3893-3900
2. https://searchfox.org/mozilla-central/rev/6a72a6d20eeb1b20b93862a79166938d6ce794a0/testing/mochitest/tests/SimpleTest/EventUtils.js#376-393,400
3. https://searchfox.org/mozilla-central/rev/6a72a6d20eeb1b20b93862a79166938d6ce794a0/dom/events/MouseEvent.cpp#81-84
4. https://searchfox.org/mozilla-central/rev/6a72a6d20eeb1b20b93862a79166938d6ce794a0/layout/base/PresShell.cpp#8987
Differential Revision: https://phabricator.services.mozilla.com/D251091
735e6414f0