[Regression] On Windows, there are missing repaints after combobox values are changed

While testing the http://releases.qt-project.org/qt5.0/beta-snapshots/2012-06-04_19-47-13/Qt-5.0-beta-offline-win-x86_msvc2010.exe binary installer, I noticed that various example applications were failing to repaint when combobox values were changed.

Steps to demonstrate the issue:

1. Launch the basic drawing example (C:\users\foo\QtSDK\Desktop\Qt\5.0.0\msvc2010\examples\painting\basicdrawing\debug\basicdrawing.exe if using the above installer).
2. Observe that the "Shape" combobox is set to "Polygon".
3. Click on the "Shape" combobox and select "Rectangle".
4. Observe that the "Shape" combobox still displays "Polygon" and the widget above it still shows polygons.
5. Maximize the window.
6. Observe that the "Shape" combobox now displays "Rectangles" and the widget above it now show rectangles.

The same problem is evident for the other comboboxes in this application, but not for the spinbox and the checkboxes (i.e. changing the Pen Width spinbox or the Transformation checkbox causes an immediate repaint).

Other examples using comboboxes also had the same problem (e.g. examples/sql/books, examples/painting/painterpaths, examples/painting/transformation).