There ARE, however, screen re-draw issues with your curtain too, Andrew.
Strictly speaking modeling, here, for a flat curved surface, I'd simply extrude an arc or polyline of the right shape to the right height from plan view. An arc will end up a half-cylinder shape when the extrude is filled, unfortunately, so using the Extract tool from the modeling toolset will let you take off just the curved shape. You can then delete the original extrude. Using polyline, though, avoids this problem.