Path: utzoo!attcan!uunet!husc6!bloom-beacon!EXPO.LCS.MIT.EDU!rws
From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: button press/release race condition, lost events
Message-ID: <8811192056.AA20497@EXPO.LCS.MIT.EDU>
Date: 19 Nov 88 20:56:36 GMT
References: <277@stan.com>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 32


    Client                              Server
    -----------------------------       ---------------------------
    Select input on ButtonPress
                                        User presses button
                                        Dispatch press to user
    Receive button press
    Grab pointer selecting input on
      ButtonRelease & other stuff
      (e.g. ButtonMotion)
                                        Process Grab
                                        User releases button
                                        Dispatch release to user.
                                        User presses button again.
                                ***     Discarded, pointer still grabbed and
                                          ButtonPress not selected.
    Receive button release.
    Ungrab pointer
                                        Process Ungrab
                                        User releases button
                                        Dispatch release to user.
    Receive 2nd release, but no
      matching press.

    The grab is not necessary, but illustrates a common paradigm.

The grab is necessary to get the kind of error you describe.  Otherwise,
the button press would have caused a grab, the event mask would not have
changed, and the grab would have been release automatically when the button
was released.  Alternatively, your bug is that you aren't using synchronous
event processing during the grab; if you had, then you would have been in a
position to release the grab before the subsequent button press was processed.