[foksi]

I have similar problem on computer with nvidia card. Landscape works without any problem but in portrait mode screen is refreshed only when mouse cursor is moving on that display. When I make screenshot it shows newest state, so the memory buffer is refreshed but it isn't visible on the display.

I don't have any problem on second computer with intel+ati - portrait mode works without freezing.

Edit:
I've installed Ubuntu 20.04 and checked what works on fresh system. Problem with freezing in portrait mode exists only on nvidia proprietary driver. On nouveau it works without freezing. So the problem is probably connected with nvidia proprietary driver. The problem isn't new. I saw a message posted almost 3 years ago with exactly the same problem described.

Workaround:
1. Switch to the nouveau driver if you need portrait mode.
2. Rotate your monitor to landscape if you need performance provided with proprietary driver.