Pmw Combobox behavior

In Fvwm 2.5.12, XFree 4.3.0.1, python 2.3.4, the program

#!/usr/bin/python
from Tkinter import *
import Pmw
root = Tk()
root.geometry("+0-40")
box = Pmw.ComboBox(root, scrolledlist_items = [1,2,3])
box.pack()
root.mainloop()

seems to learn by try-and-error where to expand the list to.

Note that each of the images displays the lower left corner of the screen which is due to the root.geometry("+0-40") (place application straight to the left and forty pixels above bottom) command.

Before clicking a mouse:
After first mouse click its the arrow button (black triangle) the combobox is trying to expand the scroll list to the downward direction (not a good idea when the window resides in the lower left corner, ouch!):
Second, third, fourth, ... mouse click on the arrow key (black triangle) makes the combo box expanding its scroll list to the upward direction (good!):

Strangely after the second mouse click the scroll down list has learned to pop up to the top, not to the bottom. Question: How to implement this behavior explicitly, so that the first mouse click already does it right? (One-click scroll list, not a two-click scroll list.)