I've built Fuse git master 9d8538c47b16adb8999632bf22f1010efbeba173 (Nov 25 2019) and started Workbench 2.1 in it configured for being controlled by kempston mouse. To my surprise, pressing middle button did not exit mouse arrow from the emulator window. I had to quit entire emulator (by pressing Alt+F4) to regain control over my host desktop system!
This is an regression, as it worked in Fuse 1.5.7.
I'm using GTK+3 UI.
Mouse arrow can exit FUSE window with Esc key... this is less comfortable solution (than middle button click) as it requires reaching for the keyboard.