Duke3D: Screen flickering on E2L11 (Lunatic Fringe)

There is a longstanding bug with overlayed Sector-Over-Sector areas where the player view is briefly transported to the wrong layer, causing screen flashes for a single frame. This can be seen in the following video at 0:16

https://www.youtube.com/watch?v=_1NfpfyOyik

Bisection indicates that this problem started with r7601 (commit 23d78a10) where some changes to clipupdatesector were made.