Comment 27 for bug 1277905

Ok, so this proves my point. The unwanted "change" somewhere in the X/GL stack is that the visual with the composite alpha channels is now "accidentally" chosen by default whereas the applications expect the alpha channel to be ignored.

I just decompiled Minecraft and hacked the calls to glClearColor to set the alpha value to "1.0" and this "corrects" the output (at least for Minecraft...).

gcc -o test test.c -lX11 -lXrender -lGL && ./test

Prints:

visual 0: alphaMask = 255
visual 1: alphaMask = 0
visual 2: alphaMask = 0
visual 3: alphaMask = 0
visual 4: alphaMask = 0
visual 5: alphaMask = 0
visual 6: alphaMask = 0
visual 7: alphaMask = 0

Which means that the first visual proposed contains the alpha mask causing unwanted alpha compositing.