2nd February 2017, 10:50
When you get into such a state, have you tried Ctrl+Alt+Backspace? It restarts X server, always helps for me. But I don't know if it's enabled by default - check in System Setting -> Input Devices -> Keyboard -> Advanced and there find the option Key sequence to kill X server. When you open it, there shouldbe just 1 option Ctrl+Alt+Backspace - enabled it and hit Apply. The next time you have the screen stuck, just hit Ctrl+Alt+Backspace and it will restart the X server and present you with a login screen as if you would have restarted. It's much quicker than waiting to reboot 