Xkill… Now that’s a name I havent heard in a long time :-)
Or Netscape, for that matter
It’s still around and kicking unresponsive GUI tools.

It’s still around and kicking unresponsive windows.

I know, but I haven’t used it since I switched to Debian ;-)

KDE has it built in to default key binding of Ctrl + Meta + Esc helpful when in Wayland as xkill only works on X and xwayland apps.