unity-2d launcher: right-click->drag->release doesn't work right

[Nathaniel W. Turner]

Scenario A:
1. Point at a launcher entry in the Unity (2D) dock.
2. Press the right mouse button and hold. (A menu appears.)
3. Drag mouse to the right to point at the first menu item.
4. Release button.

Expected Behavior:
During step 3, the first menu item is shown selected.
During step 4, the first menu item is chosen.

Actual behavior:
During step 3, the menu disappears!

Scenario B:
1. Point at a launcher entry in the Unity (2D) dock.
2. Press the right mouse button and hold. (A menu appears.)
3. Drag mouse down and to the right to point at the second menu item.
4. Release button.

Expected Behavior:
During step 3, the second menu item is shown selected.
During step 4, the second menu item is chosen.

Actual behavior:
During step 3, all the launchers in the doc scroll vertically, as though they are being dragged. The menu also usually disappears (unless the first few pixels of mouse movement were to the right, in which case the menu remains while the launchers move vertically, but no menu items are selected.)

(When running Unity (3D), the actual behavior for both scenarios matches the expected behavior.)

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: unity-2d 5.12.0-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-24.37-generic 3.2.14
Uname: Linux 3.2.0-24-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu7
Architecture: amd64
Date: Thu May 17 20:39:18 2012
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110426)
PackageArchitecture: all
SourcePackage: unity-2d
UpgradeStatus: Upgraded to precise on 2012-01-16 (122 days ago)