In VMWare and other virtualizing programs there is Ctrl-Alt key combo that releases the mouse back to windows. Not sure it would work with mir window as I had mouse stuck inside windowed mir too when I ran into a glitch while toggling from full screen to windowed mir and I couldn't release the mouse.
Before someone comes with 'windowed mir doesn't capture the mouse', well yeah, but it gets stuck when something goes wrong. Maybe it is not the same as mouse captured in full screen view but then again, perhaps it is.
At the time I opened task manager by Shift-Ctrl-Escape but mouse was still stuck inside mir window...
I believe I got it released in the end but don't remember what keys I might have pressed. Can try Alt-Tab or include shift in combination with Ctrl or Alt.