Somehow my cursor is stuck in the upper left corner of my desktop machine.
The "present windows" option for that corner is active, but it goes back and forth rapidly. Imagine a flashing of pages and apps expanding and contracting rapidly.

I don't know how to stop it as the mouse no longer controls the cursor.
I can press Alt Tab and eventually get it to hold still and pressing the up or down arrow I can hold a particular window.

If I can stop it, I think removing the default check for making the upper left corner active for Screen Edges in the Workplace Behavior option of System Settings might fix it.

I have two monitors. The cursor appears in the UL corner of the left monitor.
I am typing this note on a small laptop.

At times I have, using CTRL ALT DEL, been able to get the Leave options to display and select shutdown, but rebooting and Advanced Settings don't provide a workaround.

Note: I got interrupted by a phone call, and returned and found a stable working system; without action on my part, problem solved for now: no "present windows" or oscillation.
I unchecked the Screen Edges in the Workplace Behavior option of System Settings in the hope that will prevent the problem.

Still, if anyone knows what the issue is, and even better, how to actively recover from oscillating windows, I would like to understand it.
I think what I know so far falls short of something that is an identifiable bug.

I don't know if this is related to your issue, I have "present windows" set to the upper left corner and noticed a similar flashing if I continued moving the mouse to the upper left after present windows had been activated.

This is apparently intentional and can be tweaked. In the Screen Edges settings, changing the "Activation delay" and "Reactivation delay" settings adjusts this behavior.

I increased the number of seconds as magic, just in case.
My experience was that I lost control of the cursor. It was as if it were help by a magnet in the upper left corner.
I guess that did not happen to you.

I have no idea why it stopped this morning.