Clicking the scrollbar just scrolls slightly in that direction instead of moving the slider under the pointer immediately and starting a drag.
Looks like it was fixed in https://bugs.webkit.org/show_bug.cgi?id=210002 for GTK3, but that code path is never hit in GTK4 and we use ScrollbarThemeAdwaita directly, so it will have to be done there as well.
Created attachment 433278 [details] Patch
Committed r279821 (239582@main): <https://commits.webkit.org/239582@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 433278 [details].