This bug covers making visibleContentRect cross-platform. visibleWidth and visibleHeight should just be implemented in terms of visibleContentRect.
Created attachment 23822 [details] Patch
Comment on attachment 23822 [details] Patch + int width = GetClientSize(NULL, &width); + int height = GetClientSize(NULL, &height); I don't think this works. Otherwise, r=me.
Fixed in r36909.