Now that I think of it, system-wide idle time is available on Macs via a terminal command, or on the three platforms with a few lines of C (https://github.com/a

Now that I think of it, system-wide idle time is available on Macs via a terminal command, or on the three platforms with a few lines of C (https://github.com/anaisbetts/node-system-idle-time https://github.com/anaisbetts/node-system-idle-time). So I guess that's solvable. A system-wide popup might be harder, though.