Steps to reproduce:
1. Run HelloComboBox demo
2. Press Tab key several times to traverse focus to an editable ComboBox. Then the input caret blinks on the TextField of the ComboBox, and it seems the focus is on the TextField.
3. Invoke Chinese or Japanese input method.
4. Type soming to compose Chinese or Japanese text.
 
Problem: the composing text appears at the top-left of the screen, which means the ComboBox allows IME but doesn't handle input method events, the events are handled by the OS. (focus-traverse.png)

But if clicking mouse on the TextField of the ComboBox to get focus, it works fine, the composing text appears on the spot of the caret. (mouse-click-focus.png)

It looks like when using focus traversal keys, the focus is actually on the ComboBox itself, not on the TextField. The ComboBox itself doesn't handle IM events.