After some changes in the scrolling code WPE and GTK are not handling the scrolling thread independently when the frameDone happens, they rely on the main thread, that causes freezing in the UI.
Created attachment 414950 [details] Patch
Regarding the problem of avoiding hasProcessedWheelEventsRecently, probably Simon can help us to clarify how Apple ports are implementing the kinetic scrolling. WPE and GTK animation based on the class ScrollAnimationKinetic that inherits from ScrollAnimation. We would like to change this and make it as similar to the Apple ports as possible to make sure it is harder we break each other ports. The ScrollAnimationKinetic is moving the position using scrollTo directly using a timer, I guess that is not how your ports is doing it? Thanks for the clarifications in advance.
Created attachment 415111 [details] Patch for landing
Created attachment 415113 [details] Patch for landing
Patch 415111 does not build
Committed r270292: <https://trac.webkit.org/changeset/270292> All reviewed patches have been landed. Closing bug and clearing flags on attachment 415113 [details].