It's my understanding that the usage of the debugger is detectable by scripts running on the page. I've actually come across content websites that will refuse to activate their features (play video) if the debugger is active.
Similarly on Android some video streaming apps will refuse to load and play their content catalog if USB debugging is enabled.
A javascript file, whose name changes with every refresh, is loaded. The script calls the `debugger` keyword, which only functions when you have DevTools open.
Meanwhile, a tickers is running. If a tick takes too long, it interprets that as the debugger having been called.
This method and a few others are described in this paper: https://www.usenix.org/system/files/sec21-musch.pdf
The debugger timing method is described on page 10.
<script>
setInterval(function() {
try {
let before = new Date().getTime();
// This will pause the program execution if debugging is enabled.
// If debugging isn't enabled this statement is a no-op.
debugger;
let after = new Date().getTime();
// Detect if the program was paused or not.
if (after - before > 0.01) {
// > 10ms difference? The program was (most likely) paused by the debugger.
document.body.innerHTML = "<h1>Debugging started</h1>";
}
} catch(e) {}
}, 1000);
</script>
I think you’re probably right that it wouldn’t detect presence of your extension, since that code runs in its own context and I assume doesn’t actually call the debugger. If your extension injects a script into the page, then that could be detected with this technique or similar.