It's only implemented for mac platform, we can just move it to ThreadedScrollingTrree.
Pull request: https://github.com/WebKit/WebKit/pull/5493
Committed 255722@main (248bd5dfd870): <https://commits.webkit.org/255722@main> Reviewed commits have been landed. Closing PR #5493 and removing active labels.
<rdar://problem/101328325>