Because bug 339548 obviously needs some platform dependent parts in widget it is going to break OS/2 if we don't manage to follow it up, so I'm going to mark this blocking. (I think it's better to discuss this on a new bug than adding noise to the main bug.) Platform specific bits are currently in attachment 385578 [details] [diff] [review] and attachment 385567 [details] [diff] [review].

This patch should prove adequate if not overly robust. The platform-specific code is supposed to set a persistent clipping region for the child windows of plugin widgets. Apparently, this is possible on Win & GTK. However, on OS/2, clipping regions aren't associated with windows, usually aren't persistent, & have no effect on drawing done into children of the clipped window (which is where OS/2 plugins paint). As an alternative, this implementation uses the child windows' dimensions as a clipping rectangle, resizing them as needed. This should suffice in most situations because the clipping region is usually a simple rectangle. It will probably fail for complex regions composed of multiple rectangles.

This bug is essentially a subset of the widget removal bug 501618. Once I post a patch to it containing all OS/2 changes, this one can be closed out.

As Rich said, this will be addressed in bug 501618.