Looking at the code[0], it just defines an aspect ratio threshold (170px in either direction, on line 13) for your browser's viewport and triggers if it's outside of that on width or height. So when you open a second panel, your viewport goes back to being closer to 16:9/16:10 and the tool considers that within both thresholds.

The detection is hilariously primitive, entirely unreliable, and only knows about your devtools directly if you're using Firebug.

[0]: https://github.com/sindresorhus/devtools-detect/blob/main/in...