When using files and dragging-dropping files (while sorting files for instance), the mouse pointer regularly freezes in drag mode, and nothing seems to be working to recover control of the machine.
The solution is Alt+Tab to files app and then press Alt+F4 to close it. It releases the mouse pointer from drag mode successfully.
The question was originally solved in one of the answers to the question asked here.
It took me a while to discover how to unlock the situation, and not having to hard reboot to get out of it.