mouse cursor disappears when typing starts

With an xterm window on the screen, I move the mouse cursor into the window -- voila, focus --
so I type "ls".
As soon as I start typing, the mouse cursor disappears. It only comes back (right where I last saw it) when I move the mouse again.
(It didn't move at all during its little nap, just went invisible.)
All the expected output, from the ls command, shows up as expected, followed by a new prompt
and a nice (solid) text cursor. (I.e. the window still has focus, yay.)

Started happening today. 24.04. Happens in Gnome classic, Gnome classic on Xorg, Ubuntu, and Ubuntu on Xorg.
Obviously (I hope) the mouse cursor should stay visible most (let's say all) the time.

Did I inadvertently activate some feature I never heard of?

Happens in xterm, gnome terminal, and in any firefox text entry box (including the one I'm typing into right now).
I.e. probably any window that can be typed into.

Does not happen during login.