It seems that when you compile the server with Composite, the color tables are sometimes incorrectly initialized. Go through several server generations and you should easily see this problem. I have tracked this down to the changes in dix/colormap.c. When I compile with -UCOMPOSITE on that file alone, the problems go away. I suspect this has to do with alphaMask (and offsetAlpha) not being initialized. I see that there are some cases where they are initialized in the composite/compinit.c but that code only affects the alternate visuals. Where should alphaMask/offsetAlpha initialized for the standard visuals? And what should they be initialized to? I suspect this will cause problems on other drivers as well.
There's one hunk of diff missing for fb/fbcmap.c for COMPOSITE, but that wasn't enough. I'll take a look at this when I get back (monday) if nobody beats me.
I've eliminated the core colormap changes for Composite. That should fix this bug.
Yes, that fixed the problem. Closing.
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