http://bugs.winehq.org/show_bug.cgi?id=23098
--- Comment #10 from Roderick Colenbrander thunderbird2k@gmail.com 2010-06-14 22:44:40 --- I have just tested this on a Radeon HD3450 running FGLRX and it doesn't have the rendering bug.
The 32-bit DDB patch caused a lot of bugs like this and most have been fixed now. It exposes an area of XRender which can be buggy in drivers. Since neither Nvidia nor FGLRX have the rendering issue I'm leaning towards a driver bug.