I occasionally having this problem where when I use the spacebar to initiate a pan I cannot exit the pan system wide. When I mean system wide, I mean I cannot release the pan, and in every application I can no longer use the mouse. It's stuck in pan mode by Inkscape in every application on my desktop.
The only way to regain mouse control on my desktop is to alt-tab to a command window and "pkill inkscape".
I am using "Inkscape 91 r" on Ubuntu 14.04 64-bit with Unity as the default DE.