Comment 23 for bug 437378

... the reason seams to be that cairo(-dock?) feels like it has to set a custom color map on the root window

RGB_DEFAULT_MAP(RGB_COLOR_MAP):
                colormap id #: 0x1400001
                red-max: 255
                red-mult: 65536
                green-max: 255
                green-mult: 256
                blue-max: 255
                blue-mult: 1
                base-pixel: 0
                visual id #: 0x8c

calling "xprop -root -remove RGB_DEFAULT_MAP" sanitizes the Qt behaviour.

-> cairo(-dock?) bug. period. =P