It's not possible scroll other scrollable areas with the mouse wheel. This is because we are using the given x, y delta in the scroll signal as the position. In GTK4 the scroll event doesn't include a position, so we have to use the last motion event.
Created attachment 400730 [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 http://trac.webkit.org/wiki/WebKitGTK/AddingNewWebKit2API
Committed r262374: <https://trac.webkit.org/changeset/262374>