Looks like we can't reproduce the issue or there's a problem in the test case provided.
Ext GWT Premium Member
Combobox dropdowns cut off when screen height is shortened
I am unable to access all of the options in a combobox when my screen height (or IFrame height) is shortened to a small enough size. See the attached image of the GXT showcase, when the browser (Firefox) is compressed to a shorter height. You have the same effect when you are in an IFrame (portlets, etc..).
GXT 2 had a setMaxHeight option for ComboBox, which might provide a workaround this problem, but this is missing from GXT 3.