The current Bandpopup check when assigning vertical position is:
-If the space below the input is larger than the popup, display using after_start position. (below, toward the right, will not clip out of screen)
-If not, and if the space above is larger than the popup, display using before_start position (above, toward the right, will not clip out of screen)
-if not, display the popup using after_right, overflow true (below, toward the right, will clip out of screen.)

The same calculation can be done horizontally to display the popup using the "before_end" or "after_end" position if the popup cannot be displayed fully to the right of the input.

See fiddle for example:
http://zkfiddle.org/sample/1h1hugj/3-bandpopup-horizontal-position