If the console command "Stereo on" is used to enter Oculus mode in a standalone window, the onscreen window and Oculus viewport go black if the standalone loses window focus. Window is still running and entering "stereo off" will bring the standalone window back.
1. Open Couch Knights in 4.6
2. Launch standalone game
3. Enter console command "stereo on"
4. Press Alt+Tab to have focus switch to another window
5. Alt+Tab back to the standalone window
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-6510 in the post.