Wow, that code is entirely unreliable. It just detects your browser's viewport being outside of some threshold of a "normal" aspect ratio and then just blindly assumes that means the devtools are open. The only devtool it can actually detect is Firebug. Everything else is nothing but a blind guess based on the size of your viewport. I'd be astonished if anyone used this code for anything serious.

… that incorrectly detects devtools as open, when it isn't (false positive), and fails to detect devtools when it is open in a separate window (false negative)…

Yeah, as other posters hint at, vertical tabs. But it isn't just vertical tabs that'll trigger it, any sidebar will, including native/vanilla ones; AFAICT it's just looking at the client area being less than the window by some threshold.

Tree Style Tab is apparently detected as vertical dev tools, and opening the horizontal dev tools changes it to no. This is kind of hilarious.

Vertical tabs in Edge seems to trigger false positives on this. Really hope that's not the only heuristic they're using.

False positive in the Arc browser, no devtools open.

This is so hilarious.

It only detects if there is a block in the browser that does not serve as website rendering. Since I use Tree Tab on side panel of Firefox, this plugin will believe I have opened devtools all the time.

Edit: it will think I closed devtools when I really open devtools. I start to wonder how shitty the code is.

I just turned on responsive mode in Firefox to fool it. Definitely not reliable.

On iPhone 13 mini Safari it says I have the dev tools open. Only when I hide the toolbar it says „no“.