Given what the fix for this is, I'm not convinced this is a regression, but it has been reported as such so perhaps some other mechanism changed somewhere that caused this. At the moment, if you try to scroll via a touch-screen, the velocity with which the swipe happens is massively multiplied and in the wrong direction on the y axis.
Created attachment 448592 [details] Patch
Thanks for the patch. If this patch contains new public API please make sure it follows the guidelines for new WebKit2 GTK+ API. See https://trac.webkit.org/wiki/WebKitGTK/AddingNewWebKit2API
Committed r287794 (245852@main): <https://commits.webkit.org/245852@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 448592 [details].
<rdar://problem/87276366>