Viewport Freezes visually, but .app can still be quit. (WIP 5.2)

A few times this last modeling session, the viewport Freezes. (Perspective, full screen)
No zoom, pan, select, nothing. There is no beachball. A force quit is not necessary as the .app responds to Cmd+W. Reopening the file brings the view back to normal.

Interestingly, when the file is re-opened, the model it is spun around and flipped, as if the app had been listening to the manipulations during the freeze, but not updating the visual display.

This happened after running Extrude, but that doesn’t seem related, I can’t remember the command it followed after the first. Also, I have moved on to a child file, so I think it is happening independent of the file.

Sorry this isn’t more specific… yet.