Switching to devtools also triggers a focus change and is detectable by other means (such as repeatedly invoking a debugger statement).

One can type in devtools withouth having the focus on dev tools, but indeed, to track down the event, one has to loose focus for a while. But after you find out what line of js is needed, then you can just inject that without dev tools with greasemonkey for instance.

But probably a general solution exists ... and there are actually extensions that will do that in general.