Scrollbar in the drop-down is unusable for non-focusable choice, Win32

Scrollbar in the Choice's drop-down is not usable when choice is non-focusable. To reproduce the bug, do the following:

1. Run the attached test. A frame will open up with a button and a choice.
2. Click on the button. This will make choice non-focusable.
3. Click on Choice. The drop-down appears.
4. Without moving the mouse inside the drop-down, try clicking on the scrollbar thumb or the up/down arrows in the drop-down.

If the drop-down disappears, the bug is reproduced. It works fine when I move the mouse inside the drop-down and highlight an item before clicking on the scrollbar.

This is reproducible only on Win32 and not on XToolkit. I reproduced it on Mustang-b73.