Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!spool.mu.edu!munnari.oz.au!labtam!krite!graeme
From: graeme@krite.labtam.oz (Graeme Gill)
Newsgroups: comp.windows.x
Subject: Re: PC X servers / backing store
Summary: Why does "backing store always" break clients ?
Message-ID: <10073@krite.labtam.oz>
Date: 5 Mar 91 03:39:16 GMT
References: <9103032356.AA00215@devnull.Eng.Sun.COM>
Organization: Labtam Australia Pty. Ltd., Melbourne, Australia
Lines: 33

> 	"A backing-store of WhenMapped advises the server that maintaining
> 	contents of obscured regions when the window is mapped would be
> 	benficial.  A backing store of Always advises the server that
> 	maintaining contents even when the window is unmapped would be
> 	beneficial.  ...  While the server maintains contents,  exposure
> 	events will not normally be generated,  but the server may stop
> 	maintaining contents at any time."


	I vaguely  remember seeing a discussion about this recently somewhere,
but could someone throw some light on why setting backing store to "always"
as opposed to "not useful" or "when mapped" breaks many clients written
in the standard fashion. What seems to happen is that since the backing pixmap
is created on window creation, then raising the window doesn't generate an
expose, so the client never draws anything to the window.
	In other words, the server seems to go to extremes in avoiding sending
expose events to the client, to the point where it is not initialising the
backing pixmap ! This seems like a bug in the server backing store code.
Is this behavior supported so that a client that sees that backing store
is always enabled can assume it will never get an expose event, and that it
is ok to draw to the window prior to raising it ? This seems contrary to the
quoted protocol section above.
	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.

	Graeme Gill
	Labtam Australia Pty. Ltd.
	graeme@labtam.oz.au