Edge condition in idle monitor

Binary package hint: gnome-session

While investigating https://bugzilla.gnome.org/show_bug.cgi?id=625374 I've run
across a corner case in the gnome-session idle monitor which also exposes the
Xserver bug referenced in that gnome-screensaver bug.

In _xsync_alarm_set in gs-idle-monitor.c the positive and negative transition
intervals are set to the same value. However, the SYNC extension defines the
positive transition as set when the counter goes from strictly below the
threshold to greater than or equal to the threshold and similarly a negative
transition is triggered when the counter goes form strictly greater than the
threshold to less than or equal to the threshold.

Thus in the current set up there's a chance that the positive transition can
trigger, marking the session as idle, and some user input occur on the same
click so the IDLETIME count will hit the threshold but not go above so the
negative transition will not trigger. Thus the session will not be marked as
active.

The negative transition threshold should be set to 1ms less than the positive
transition to ensure that it always fires.

ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: gnome-session 2.31.6-0ubuntu1
ProcVersionSignature: Ubuntu 2.6.35-18.24-generic 2.6.35.3
Uname: Linux 2.6.35-18-generic x86_64
Architecture: amd64
Date: Wed Aug 25 11:13:38 2010
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Alpha amd64 (20100817)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_AU:en_GB:en_US:en
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/usr/bin/zsh
SourcePackage: gnome-session