X server bug drawing XOR rectangles (3.3 on PI)

[Guido van Rossum]

[I have no idea where to mail this (nor does our local SGI guru) so I'll
post it here and hope someone at SGI will pick it up.]

The 3.3 Xsgi is definitely a vast improvement, but I once crashed it and
got it to behave weird with this bug:

I have a program (really a library -- stdwin if you're interested) which
creates a tall window, say, 100x500, with an even taller subwindow, say,
100x2000.  Using scroll bars I move the subwindow up and down in the
main window and draw in it (in the subwindow, that it).  This goes fine.
But I want to invert a particular line of text.  This I do with XOR
drawing mode and the bitplane set to the XOR of the foreground and
background pixel values (I draw B/W only).  When the XORed rectangle is
visible in the main window, it is fine.  When I scroll it a little
bit out of sight, it is still fine.  But when I scroll it way out of
sight (outside the screen boundaries?), the entire main window gets
inverted.  This is consistent; when I issue the same XOR instruction
again the window is inverted back to normal.  Note that if I make the
width of the inverted rectangle much larger than the window (1000) I may
crash the X or NeWS server (this happened the first time when I tried
it).

I suspect that the outcome of the clipping applied by the server is
wrong, but I can only guess.  I can provide an example if required.

--
Guido van Rossum, Centre for Mathematics and Computer Science (CWI), Amsterdam
guido at cwi.nl or ..!hp4nl!cwi.nl!guido or guido%cwi.nl at uunet.uu.net
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:-)