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.