I used to work with window in the past, and when Windows was stuck (cursor doesn't move or respond to clicks, for example), I could just clicked Ctrl+Alt+Delete, open the task manager and end the task that causes the OS to stuck.
Unfortunately, this feature doesn't exist in Ubuntu. I know the command 'xkill', but it doesn't always work, and in order to use it, I first have to open a terminal. When Ubuntu is stack, sometimes it's not possible to open a terminal. I also know the commands 'lsof' and 'kill -9 ...'. Still, it's not always useful when OS is stuck.
Do you know any technique that is as effective as ctrl+alt+del in Windows?