Use the attached test to reproduce the issue.
When an application window appears on the screen the window position gets changed several times (see the console output). This is fine.
But sometimes when you try to resize the window horizontally (or vertically) you can see in the console that the window position listeners reporting that the position gets updated which shouldn't happen - window stays at the same place, just resized.
This affects ComboBox control as well. You will see that the PopupControl is torn apart from the ComboBox itself. 
This issue is not always reproducible, better when launched in NetBeans.

There is a workaround (commented in the code in windowProperty change listener).