Currently, interior frame scrollbars are native widgets, which leads to a lot of mysterious code in ScrollbarGtk. It also means that for WebKit2 mouse click need to be routed through GTK+ instead of directly through WebCore. The first step in making fully-fake interior scrollbars is to complete ScrollbarThemeGtk via the Mozilla theming API. The bug tracks that work.
Created attachment 65191 [details] Add theme-based painting for GTK+ scrollbars
Comment on attachment 65191 [details] Add theme-based painting for GTK+ scrollbars LGTM. The only thing that looks problematic to me is this: 56 // We do not support multiple steppers per end yet. 57 if (part == BackButtonEndPart) 58 return IntRect(); Though we don't seem to be regressing from our previous state, and thankfully themes sporting more than one button per side seem to have gone out of fashion =).
Committed r66148: <http://trac.webkit.org/changeset/66148>