If you click the popup arrow to show the GtkCombo's popup list, then move
the mouse slightly while still holding the button down, it selects the
first element in the list.

It is easy to do this accidentally when clicking the button to show the
popup.

(This bug is also in 1.2.x)

This is related to the fact that you can select an item with a single
click and drag. So as soon as the popup is shown it goes into selection
mode, and if you move the mouse it selects the closest item.

One possible solution would be to add a flag 'starting_drag' which you
set when the button is pressed and the popup is initially shown.
Then you ignore mouse motion events until the mouse enters the list window,
at which point you reset 'starting_drag' and start selecting items.