Hard to explain this one ...
Sometimes the last line of the screen in a direct-rendering mode looks funky. As an object passes over it, colours get written in, but selectively, as though a Z-buffering algorithm were being applied or something.
Certain xlock modes show this quite clearly e.g. gears, where now again one or more of the gears touches the bottom of the screen, or the 3D bubbles where a streak of colour is left at the bottom.
Now, when the screensaver mode changes, sometimes when another GL mode kicks in, the garbage in the bottom line is still there, but others will clear it correctly e.g. pipes.
Very occasionally I've seen a similar effect in the rightmost column of the screen, though I havn't seen it often enough or peered hard enough to see if it's just garbage caused by an off-by-one clear area, or if it is behaving in the same odd way as the bottom line.
I'm convinced the bottom line is somehow being treated as both part of the display and part of the depth buffer, but have no way to verify this. I don't know enough to code up tests for that yet :( although I could/can probably figure out in time.
/me makes note to learn GL in his alleged copious quantities of free time.
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