>> I keep thinking of writing something that detects the top-most app window and draws an obvious box around it.

I would use this in a heartbeat. With Windows 10/11 I usually have the option to apply a garish accent color to the active window active. Nowadays, more and more apps don't use native window frames anymore, so that option works less and less.

The W11 task bar with its barely legible indicators doesn't help either.

On a big ultra-wide display with a few windows open, I sometimes struggle to see which one is active.