[GTK3/Firefox] Fix scrollbar click region
Summary:
Scrollbars now have a click region equivalent to their visual space in Firefox.
FIXED-IN: 5.17.3
BUG: 413118
Test Plan:
Before: 5px edge on scrollbars doesn't respond to clicks.
After: 5px edge on scrollbars responds to clicks.
Reviewers: VDG, Breeze, ngraham
Reviewed By: VDG, Breeze, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25180