I am curious if the Alt + F2
xkill command has been replaced by something similar, or if it has been removed entirely.
I know we can use other methods to stop processes, I'm just curious.
This looks like a bug under x11 in 18.04. I've tried this in at least 3 systems before landing to this conclusion.
xkill if run in a "Run Command" dialog which appears after using Alt+F2 seems to occasionally don't actually execute the command and return "the cursor" instead of X (cross sign). I haven't found a bug report yet and still searching for it.
As of now you can switch to "Wayland" and
xkill seems to work normally under Wayland. To switch to wayland, click on gear icon and select "Ubuntu on Wayland".