When the game's window mode is set to borderless, the game doesn't actually cover up the entire screen, even on native resolution. Here's a screenshot:
Reproduction
Go to the game's settings, then Video.
Set the window mode to Borderless, then click Apply.
Compatibility Report
System Information
I confirm:
steam-2379780.log
Symptoms
When the game's window mode is set to borderless, the game doesn't actually cover up the entire screen, even on native resolution. Here's a screenshot:
Reproduction