Not yet mentioned but might work in some cases - try CTRL+ALT+RIGHTARROW or LEFTARROW. This did switch workspaces, and doing so somehow unhitched whatever was blocking mouse and keyboard actions. I could then switch back to the first workspace, kill Chromium, which I had by then figured had crashed and was hogging all keyboard and mouse events. Now everything's back to normal, no reboot or loss of anything involved.
In my case, just minutes ago, everything was hung except the mouse moved the cursor, but the mouse did no more. The PrtScn made a popup saying I could drag a rectangle (but not) or hit ESC (works). Nothing else worked. Not even moving the cursor onto another window, when I have focus-follows-mouse, changes the window frame highlighting. But Ctrl+Alt+F3 did switch to a text term, Ctrl+Alt+F1 brought back the X windows session.
The culprit was Chromium. I was reading, switched tabs in one browser window, and upon clicking the tab everything froze but for the few things just described. Switching workspaces