Please describe the problem:
When activating a combo box whose items are too numerous to fit the space left
below it (down until the bottom of the desktop), the items are wrongly
positioned on the popup menu. It is as if the popup menu shown has the correct
dimensions, but the items are not rendered beginning from the top of it, causing
an empty area to appear on the top and the little arrow to allow scrolling the
items at the bottom.

Steps to reproduce:
1. Add a ComboBox widget to a window.
2. Enter a considerable number of items for it; 20, say.
3. In your application, use libglademm to load the widget.
4. Upon runtime, try activating the combo box with its parent window positioned
at different heights with relation to the desktop.


Actual results:
Depending on the position of the combo box widget with regards to the desktop,
its items will appear vertically displaced when the combo box is activated.

Expected results:
The combo box's items should occupy the whole area of the popup menu and
scrolling arrows should be displayed only if necessary, that is, if the total
number of items were so large that they would not fit in an area as large as the
desktop's height.

Does this happen every time?
Yes.

Other information:
I have taken 2 screenshots of my application where the problem is depicted:

  1) The dialog box containing the combo box widget
     http://zunino.eti.br/tests/gtkmm/comboboxtext/comboboxtext-1.png
  2) The combo box having been activated; notice the blank area at the top.
     http://zunino.eti.br/tests/gtkmm/comboboxtext/comboboxtext-2.png

Note: There is actually another issue, which is that the items entered in Glade
are being duplicated (two per line), but I won't mix things up. In any case, I
would appreciate if someone knew the reason and dropped me a line.