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From: dshr@eng.sun.COM (David Rosenthal)
Newsgroups: comp.windows.x
Subject: Re: PC X servers / backing store
Message-ID: <9103051609.AA02015@devnull.Eng.Sun.COM>
Date: 5 Mar 91 15:58:10 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 33

> 	Correct behavior should be to send an expose event to initialise
> any un-initialised portion of the backing store pixmap, whenever the window
> is raised. (This would be to prevent breaking any clients that only expect
> expose events when the window is raised.) This would also cope properly
> with a situation in which memory for the backing store pixmap becomes available
> sometime after a client has started. The backing store could be assumed to
> have been initialised if written to prior to being raised.
> 
Correct behaviour is defined by the protocol spec (page 474 of the Digital
Press 2nd edition):

	"This event [Expose] is reported to clients selecting Exposure on
	the window.  It is generated when no valid contents are available
	for regions of the window,  and either:

	-	the regions are visible,
	-	the regions are viewable and the server is (perhaps newly)
		maintaining backing store on the window,
	-	or the window is not viewable but the server is (perhaps
		newly) honoring [the] window's backing-store attribute of
		Always or WhenMapped.

What this means is that you should select for Exposure in the CreateWindow
if the backing-store attribute is Always,  because the server will
generate the Expose immediately and if you aren't listening it will be
lost,  and never de-select Exposure on a window that has backing-store
Always,  because the server may decide to start maintaining it at
any time.  This is good practice if the window has backing-store
WhenMapped,  because otherwise there are potential race conditions
between the map/unmap and the select/deselect of Exposure that you are
unlikely to get right.

	David.