This is the strangest problem I've seen and it only happens with steam.

Am using a double monitor setup (Odyssey G7 32" and a vertically mounted FI27Q 27")
I usually have my sleep settings auto-turn off the screen also, since I usually keep my computer idle for long periods of time.

But whenever I go back to my PC and try to open Steam it is just a black, empty screen that I can't use.

Reproduction:
1. Open Steam in Desktop mode
This is what it looks like: https://imgur.com/dPFe7Du
2. Lock computer, wait for monitor to auto turns off
3. Open computer again
4. Steam is now a black screen window and I cannot access anything:
https://imgur.com/AMJyRT7

The only workaround I was able to do was to go to the taskbar -> right click -> open big picture
Steam restarts and I can see the Big Picture UI now.
If I exit from this state it properly shows the screen again: https://imgur.com/a/Dg74799