cursor bug: where to look?

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've been running X on my i386 laptop for some time - the X built from
the xsrc that shipped with 4.0.1, that being the OS - and it's been
working fine.  But I recently tried X on another i386 machine, and it
exhibits an annoying cursor bug.  I have a program that uses a cursor
created with XCreatePixmapCursor with a 3x15 image and a mask of None.
This works fine on many servers.  But on some, it extends the cursor to
the right with background pixels to a width of about 16 pixels.  If I
have the program use an explicit 3x15 mask full of 1s it works fine,
demonstrating that there is no real need to mangle the data this way.

The X servers on the two machines are built on the same arch (i386)
from identical source trees, and thus are "the same", so clearly it's
got something to do with a difference between them.  The most likely
culprit strikes me as being the display-specific code, but I have
little idea where to even start looking.  Does anyone know enough about
the guts of the X server to point me in a useful direction?  (I can
probably work it out, but a few pointers could potentially save me a
lot of fumbling around.)

I can of course provide more information, if anyone can tell me where
to get it.  If this were MIT X, I'd be on familiar ground, but since
this is i386, there seems to be little choice but to drink the XFree86
koolaid.

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