Please describe the problem:
In a read-only combo, when the popup list is displayed close to the bottom of the screen, blank items are created to fill the top part of the popup list. 

Steps to reproduce:
1. open any dialog with a combo box (eg. the preference page of Nautilus file browser)
2. move the dialog so that the combo is near the bottom of the screen
3. select the combobox to display the popup list


Actual results:
The popup list shows the items of the combo box but the first few lines are blank and the first item is displayed part way in the list.

Expected results:
All the items should be displayed in the popup list without the blank items.

Does this happen every time?
Yes.

Other information:
The popup list is forced to move up the screen to avoid from being clipped (going off the screen) but it displays blank items at the top instead of the actual items.