xflock4 fails to lock screen when mouse pointer in certain places

In XFCE, when my mouse pointer is hovering over a drop down item from the menu bar (such is wireless networks, or the audio controls), xflock4 no longer locks the screen after the specified delay. It doesn't lock the screen either when invoked from a command line although it prints a success message: "xscreensaver-command: activating and locking.".

It seems to think it did though, as an attempt to lock the screen again (after moving the mouse to a different place) yields: "xscreensaver-command: already locked." even though the screen is clearly not locked.

This is a pretty massive security hole (though not a remote one) as, if I walk away from my computer while the mouse pointer is in the wrong place, the screen won't lock and anyone can sit down at the computer and take control regardless of how much time has elapsed.

Attaching screenshot.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: xfce4-utils 4.8.3-1ubuntu2
ProcVersionSignature: Ubuntu 3.2.0-27.43-generic-pae 3.2.21
Uname: Linux 3.2.0-27-generic-pae i686
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu11
Architecture: i386
Date: Sun Jul 29 05:39:30 2012
InstallationMedia: Xubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
ProcEnviron:
 TERM=xterm
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: xfce4-utils
UpgradeStatus: No upgrade log present (probably fresh install)