Keyboard input stops working after Alt+Tab

I’m using Upverter in Firefox on Windows 10. When I Alt-Tab over to another application, when I come back, my keyboard shortcuts no longer work. I can’t Space-Drag to move, Esc to switch to select, Del to remove a part, etc. I’ve tried to Right Click and bring up a context menu to bring the webapp back into focus; regular click for the same thing; clicked different browser tabs and back; none of these seem to regain focus for the keyboard input. I have to reload the page to work.

I use keyboard shortcuts over mouse actions whenever possible - thus I hit Alt-Tab a lot. This is very frustrating. Does anybody have a solution? Can others test to see if this happens to them?