it's impossible to implement a canvas context without using internal APIs currently, as the contexts do their own frame invalidation. This moves that stuff into a private method on the canvas element interface, so that there are no issues. The gfxASurface un-inlining needs to happen because of different method signatures due to the usage of gfx*String; these need to be fudged in any context implementations that are outside of internal string API usage, but that's an easy hack.

The new API looks great to me. My canvas backend is here: http://pyrodesktop.org/svn/trunk/pyrodesktop/compzilla/compzilla/src/compzillaRenderingContext.cpp Is the rect passed to InvalidateFrameSubrect in pixels or twips?

checked in. The rect is passed in screen pixels; I'll add a comment to that effect.