I've been using xterm on a KDE desktop for many years, on one Debian/Ubuntu/Arbian release after another. I use the focus-follows-mouse desktop setting, and ":set mouse=a" in .vimrc. Recent releases introduce a misbehavior I don't know how to track down. Seems to have happened Debian 10->11, Buster->Bullseye.
With vim in input mode, when I move the cursor out of the xterm where vim is running (the window loses focus), there is a beep and vim switches to command mode, as if someone had hit esc in that window. The old behavior was to wait quietly until the focus came back on the vi window, so I could paste in whatever I went to the other window to copy.
gvim on the desktop doesn't have this problem.
It happens on an old (buster) desktop talking via ssh to a shell+vim on new (Bullseye) systems.
What's going on here? Is the new shell passing along a signal it used to trap? How to track down?