This bug can be reproduced with the combo in testgtk/entry. Gtk+ version 
is 2.0.6

When the combo entry is set to a value and the user clicks on the button 
(to the right of the entry) for the first time, the value jumps to the 
first item in the list.
On later clicks, this happens when the mouse is dragged (with the mouse 
button pressed) *within* the button.

The expected behavior would be to change the value only when the user 
selects an item from the list.
The bug prevents the user from looking at the choices without changing 
anything.