A strange box has started appearing on my old Mini's screen.

A strange box has started appearing in the middle of my old Mini's display. It looks like this:

My old Mini is a Mac mini (2011) running macOS Sierra Version 10.12.6. It has been ineligible for OS updates for years.

This box started appearing late last year, long after its last OS update.

The box disappears by clicking the x in the upper left corner. It will also disappear with some kinds of unrelated mouse/desktop activity.

After disappearing, the box reappears after a long time -- hours? -- of inaction.

The box does not interfere with my mini's function as a webserver, nor as a remote fileserver from another Mac.

The Mini normally has no display, mouse nor keyboard. I usually control it via Apple Screen Share. A Bluetooth mouse and keyboard are paired to it but usually turned off so that the cat cannot control it.

However, once it appears, the strange box can be seen even when I do hook up a display directly.

What is this box?

--Gil