You create this pseudo function:

1. setTimeout to call an endpoint that will block your ip. It's set to run in 2 seconds from now. 2. Insert: debugger; 3. Clear the timeout.

Now you run this function as an interval. If the devtools is closed, debugger will be ignored and the call to this endpoint will never happen. But if it's open, the debugger will stop and the timeout will fire. Not sure if you need to patch SetTimeout to continue running while you stop but I hope you get the general idea