Step to reproduce
1) Run the attached test case
2) Resize the JInternalFrame which contains JComboBox.
3) Click any where on the InternalFrame . If you that JComboBox popup opens then the bug is reproduced.

Note: I just modified the code by adding a JPanel and adding combobox in it. When clicked any where on the JPanel JComboBox popup was not opened. So this has to be done with JInternalFrame.