Change made by setButtonCell on ComboBox is not propagated immediately. New cell becomes used only after change in selected item has been made.
This results in the following behaviour
1) calling setButtonCell on new ComboBox when nothing is selected -> default cell is still visible
2) setting cell to different one -> visible only after change in selected item
Tested on 7u60, 8u5, 8u20_b17
This results in the following behaviour
1) calling setButtonCell on new ComboBox when nothing is selected -> default cell is still visible
2) setting cell to different one -> visible only after change in selected item
Tested on 7u60, 8u5, 8u20_b17