gnome-screensaver locked screen leaks text to underlying windows

This happened after my 5y old daughter hammered the keyboard with random keys, so I literally have no idea how to reproduce it.

On the lock screen, I was unable to type anything, as if I'd pressed enter but the password was still being validated. Then I hit alt-tab and saw compiz's ring switcher activate. I tried to type my password a couple of times but the screen didn't unlock. Eventually I did switch user and finally managed to log in.

When I looked at the terminal, I realized what I was suspecting moments before. My password went through the lock window and ended up on the terminal (and also on Firefox in a Google search form).

> gimp: GEGL-WARNING: (../gegl/buffer/gegl-tile-handler-cache.c:1076):gegl_tile_cache_destroy: runtime check failed: (g_queue_is_empty (&cache_queue))
> EEEEeEeek! 1 GeglBuffers leaked
> CABBDCDDmypassword
> CABBDCDDmypassword: command not found
> [1]+ Done gimp vlcsnap-2021-09-21-17h03m59s833-2.xcf

I think this should definitely not happen, now the problem is understanding how it happened. This is my version of gnome-screensaver:

ii gnome-screensaver 3.6.1-11ubuntu4 amd64 Screensaver and screen lock formerly used in GNOME

Ubuntu is 20.04 but it's not a fresh install. Please tell me how I can debug this further, looks like a fairly nasty bug.