Need to implement the feature added in bug 218764 with UI-side compositing. See ThreadedScrollingTree::willSendEventToMainThread(), ThreadedScrollingTree::waitForEventToBeProcessedByMainThread().
<rdar://problem/103969246>
Created attachment 465271 [details] WIP
Pull request: https://github.com/WebKit/WebKit/pull/12154
Committed 262375@main (6151efcb38ff): <https://commits.webkit.org/262375@main> Reviewed commits have been landed. Closing PR #12154 and removing active labels.