screen saver doesn't start if a menu was open

Binary package hint: gnome-screensaver

How to reproduce:
1-configure screen saver preferences as follow (for testing): blank screen , 1 minute, activate screen saver, lock screen.
2- open a menu or lift click the small arrow on Firefox address bar so that the list web sites pop out.
3- wait more than 1 minute.
Expected behaivor:
 screen saver starts and the screen locked.
What happened instead:
1- screen saver never started.
2- sometime the screen saver/ lock screen start if I return to the computer. for example , while the address bar is open I move the mouse then left click the space to the right of the help menu, beneath the window title bar.

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: gnome-screensaver 2.30.0-0ubuntu1
ProcVersionSignature: Ubuntu 2.6.32-19.28-generic 2.6.32.10+drm33.1
Uname: Linux 2.6.32-19-generic x86_64
Architecture: amd64
CheckboxSubmission: ec068ef8e627860f023118eb128d8949
CheckboxSystem: fd6c484c0b5d20a2f1cd036ef0f63975
Date: Mon Apr 5 04:08:10 2010
GnomeSessionIdleInhibited: No
GnomeSessionInhibitors: None
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-screensaver
WindowManager: compiz