Create a ComboBox with a ListStore which contains several elements (A, B, C..). Start typing the name of A into the ComboBox field and select it with the drop down list and move the focus to a different Element than the ComboBox. ComboBox.getValue() will return "null" and also ValueChangedEvent isn't fired.
When following the description above, TriggerFieldCell receives a different Context.getKey() object in TriggerFieldCell.onBrowserEvent() and creates a new FieldViewData object with "lastValue" and "curValue" set to the same value and no value change is recognized on ComboBoxCell.triggerBlur().
ComboBoxCell.onSelect() is triggered correctly. Overwrite it to retrieve the selected Element.
I'm not finding that I can reproduce this issue in development mode in Firefox 10. Do you have a short test case implementing EntryPoint that you can post? Also, are you performing each selection using the mouse or keyboard? Finally, what browser are you using?