http://bugs.winehq.org/show_bug.cgi?id=14975
--- Comment #23 from Roderick Colenbrander thunderbird2k@gmx.net 2008-09-20 14:49:50 --- I did some tests on various windows opengl drivers and tested what the same ChoosePixelFormat call wow makes returns, so one with colorbits=32 and the other bits set to 0 and DoubleBuffering, DrawToWindow, SupportOpenGL and RGBA. In general it returns a format with r=g=b=8, alpha=0 and further the flags that were requested and so now and then even depth and stencil. I have the impression that it might not be the ChoosePixelFormat call which is the problem (unless the driver is doing some evil fixups).
I have the impression that the display driver is doing some evil fixup or so OR it could be that wglChoosePixelFormatARB selects a format with alpha. You would have to hack the code to see whether it is the alpha which matters for the window or for the pbuffer (it selects pixelformat 1 for the window and 9 for a pbuffer).