QComboBox text is not drawn correctly when first poped up.

This issue is most noticeable on Windows based platform but still is present on Qt X11.

Steps to reproduce
1. start the styles example
2. click the "Styles" QComboBox and change the value to "cleanlooks"
3. see that there is excessive white space at the bottom of the list (see attached snapshot)
4. see that this issue is disappears when next the "Styles" QComboBox is poped up.

This issue is present in Qt 4.6.x, SHA1 c74a5ae953899b9109ef56b2057b094152616480