Tests using `eventSender.mouseScrollBy()` don't work with UI-side composting because we need to wait for UI-side state to update.
<rdar://problem/104058894>
Pull request: https://github.com/WebKit/WebKit/pull/8429
Committed 259295@main (d5823dca0a47): <https://commits.webkit.org/259295@main> Reviewed commits have been landed. Closing PR #8429 and removing active labels.