During the 3.19 GTK+ release cycle, the GTK+ css system was reworked, making themes and programs rendering themed widgets, incompatible with the new system. We were trying to fix our rendering every time GTK+ broke something, but we were just changing whatever it was needed to make our rendering look like current GTK+ with the default theme Adwaita. This means that our rendering will be broken for other themes or that changes in Adwaita can break our rendering. This solution was good enough to ensure WebKitGTK+ 2.12 looked good with GTK+ 3.20, but it doesn't work in the long term. We need to ensure that our theming code honors the new GTK+ CSS properties (max-width, min-width, margin, padding, border, ...) in all the cases, not only the cases where Adwaita uses them like we currently do.
Created attachment 275871 [details]
Something similar is needed now for ScrollbarThemeGtk
Committed r199292: <http://trac.webkit.org/changeset/199292>