in the dialog "Create Partition" the comboxbox "Partition Type" doesn't always open

When a user creates a second partition during custom partitioning, the second time the user opens the dialog box "Create partition", the combox "partition type" doesn't open the first time the user clicks on it.

This is only reproducible in "Try Ubuntu" mode not if the user selected "Install Ubuntu" in ubiquity-dm. When it happens arrows are displayed at the top and bottom of the combo (see screenshot) so it might be a compiz or gtk bug.

This is a minor annoyance for a human, but a real blocker for automated tests and very difficult to workaround.

TEST CASE
1. Boot a desktop ISO, select "Try Ubuntu" and proceed to the custom partitioner ("Something else")
2. Create a new partition table
3. Double click "Free space" to open the dialog "Create partition"
4. Click on the combo "Use as:"
5. select "Ext4"
6. Press "Cancel" to dismiss the dialog
7. Repeat steps 3 to 6 a second time

ACTUAL RESULT
The second time steps 3 to 6 are executed, the 1rst time the user clicks on the combobox it opens and closes immediately and the user has to click a second time to open it and keep it opened.

EXPECTED RESULT
The combobox opens every time the user clicks on it

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 3.13.0-18.38-generic 3.13.6
Uname: Linux 3.13.0-18-generic x86_64
ApportVersion: 2.13.3-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Mar 21 15:18:02 2014
InstallationDate: Installed on 2013-09-03 (198 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Alpha amd64 (20130902)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)