Comment 2 for bug 1427187

this can be made to act as expected in this case by changing:

http://bazaar.launchpad.net/~ubuntu-sdk-team/ubuntu-ui-toolkit/staging/view/head:/src/Ubuntu/Components/Themes/Ambiance/1.2/ComboButtonStyle.qml

line 32 to:
defaultDropdownColor: Qt.rgba(0, 0, 0, 0.05)
and line 110 to:
color: mainButton.defaultColor

seems waiting for combo.expanded to be evaluated is the issue.

i'll try digging into ubuntu-ui-toolkit and see if get a proper fix for them.