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From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: Porting server to TMS34010 coprocessor
Message-ID: <8903171917.AA16536@EXPIRE.LCS.MIT.EDU>
Date: 17 Mar 89 19:17:19 GMT
References: <12315@swan.ulowell.edu>
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    Therefore, I need to know beforehand whether I am dealing with a
    displayed window or not.

The drawable's type will tell you this (DRAWABLE_WINDOW vs. DRAWABLE_PIXMAP).
You can even use this information at ValidateGC time to swap in the appropriate
functions.

    Is an UNDRAWABLE_WINDOW still on the display, and is INPUT_ONLY?

Yes.  You should never see an UNDRAWABLE_WINDOW passed to a graphics request.

    Also, how
    does X deal with overlapping windows, and where is the code for this? 

Deal with in what respect?  In terms of clipping, see the horrors in
server/dix/window.c and server/ddx/mi/mivaltree.c.