git-gui: Context menu doesn't work

When I right-click on the diff view I expect to see a context menu with positions like 'Stage lines for commit', 'Stage hunks for commit' etc.
Instead, sometimes, I see a short flick matching the size and position of the context menu.

Restarting application, or the whole system didn't help. All packages are up to date.
The problem manifests itself every time I right-click the diff view.

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: git-gui 1:1.7.4.1-3
ProcVersionSignature: Ubuntu 2.6.38-11.50-generic 2.6.38.8
Uname: Linux 2.6.38-11-generic x86_64
Architecture: amd64
Date: Wed Dec 28 18:20:01 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu-Server 11.04 "Natty Narwhal" - Release amd64 (20110426)
PackageArchitecture: all
ProcEnviron:
 SHELL=/bin/bash
 PATH=(custom, user)
 LANG=en_US.UTF-8
 LANGUAGE=en_US:en
SourcePackage: git
UpgradeStatus: No upgrade log present (probably fresh install)