Following up to myself...

> X is another story.  It's always aligned with the upper left corner
> of the screen, but its idea of the layout of video memory is quite
> different from the real layout, except at 1024x768 and 1280x1024.
> Incidentally, those are the only resolutions where the width is the
> same as linebytes.  Coincidence or not?  I'm still trying to find out
> exactly how X pixels map to screen pixels at the broken resolutions.

I finished testing X.  And as I suspected, it doesn't know abou the
gaps that are required in video memory when linebytes is greater than
the true width.  (Suppose the width is 640 and linebytes is 768.
Pixels (0, 0) through (639, 0) will be in the correct place.  Pixels
(0, 1) through (127, 1) will be invisible.  Pixel (128, 1) will be
directly under pixel (0, 0).  And so on.)

-- Derek