The problem is that ScrollingTreeNicosia::scrollingNodeForPoint() is returning an overflow node containing the given point, but its event region doesn't contain the point.
Pull request: https://github.com/WebKit/WebKit/pull/5756
Committed 256004@main (d911d7875a49): <https://commits.webkit.org/256004@main> Reviewed commits have been landed. Closing PR #5756 and removing active labels.