ComboButtonStyle style artifacts

from bug:
https://bugs.launchpad.net/ubuntu/+source/unity8/+bug/1427187

Test case.
- Run the wizard from unity8 project.
- Go to the language selection page.
- Tap to open the list.
- Select a language (e.g Spanish).

Expected result.
- Language is selected and dropdown widget doesn't change its style.

Actual result.
- After language is selected, widget changes its style. For a moment, the list is no longer transparent, but gray.

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

specifically line 32:
defaultDropdownColor: combo.expanded ? Qt.rgba(0, 0, 0, 0.05) : defaultColor

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

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

will give desired results in the wizard, tho not a proper fix.