Half-screen, horizontal

When I walk away from my iMac Monterey, upon return the screen is half black. The upper half (horizontally) is solid black, and the lower half shows the upper half of the sign in screen. I can type in my password, tho I cannot see the type box, and it comes up full screen and operational.

Nothing major has happened lately, and the machine is unchanged otherwise.

Searches discuss other screen filling problems, but none like this.