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From: rws@expo.lcs.mit.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: PC X servers / backing store
Message-ID: <9103051342.AA21476@expire.lcs.mit.edu>
Date: 5 Mar 91 13:42:36 GMT
References: <10073@krite.labtam.oz>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 20


    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.

Because the exposure event is generated when the window is created, and
many applications don't select for events as part of CreateWindow, they
select for events in a separate protocol request.  At least, this is what
happens in a correct server.  It's possible you are using a broken server
that fails to generte exposure events at create time.  Note that if you
mean you are using some kind of server config option to force Always on
unsuspecting clients, this is contrary to the specification, so you
shouldn't be surprised if clients break.  WhenMapped is the strongest
implicit default you can force on clients.

    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.

Nope.  Read the protocol spec (CreateWindow, para 12).