See https://bugs.webkit.org/show_bug.cgi?id=115363, this is an issue with non-native scrollbars now.
When you click on the trough outside the drag handle, the position just jumps a bit into that direction instead of:
1. Scrollbar handle jumping there immediately (in GTK even with an animation)
2. Starting a drag
Yes, we did this on purpose, following the behavior of other browsers instead of GTK, but I agree we should follow GTK for consistency, at least in the GTK port. I'll fix it.
Created attachment 396394 [details]
Created attachment 396395 [details]
Committed r260070: <https://trac.webkit.org/changeset/260070>