If a combobox is placed at the bottom of your screenspace, it will not draw upwards unless in full-screen. Anything less than full-screen, and comboboxes at the bottom of your screenspace will draw them selves down and outside of the window and will get cut-off at the windows defining edge.
Hello Godling,
I was able to reproduce the issue that you described and I have written a report ( UE-6702). It has been sent to the developers for further review. I will provide updates with any pertinent information as it becomes available. Thank you for your time and information.
Make it a great day.