--geometry=1800x1450 =< only to repare one mistake , versus my
I can add one more bit of information. I have seen these visual artificacts with the default menu coloring scheme. If you switch to the alternative menu/gui style (Shift-F11?) then these artifcacts go away ... so maybe we are inadvertently leaving some sort of alpha blend mode on after drawing the menus which is messing up the sky which is the first thing drawn in the next frame???