> I'm surprised nobody has thought of this sooner

Firefox already has a Responsive Design Mode [0] that does this, but for one pane. Combine that with a tiling window manager and you get everything except the pane mirroring without having to use a proprietary browser.

I'd be interested in knowing if any other solutions exist just to add in the syncing functionality.

Edit: a comment mentioned Browsersync [1]. Browsersync + a proper window manager should give you everything Polypane offers and more, with any browser.

[0]: https://developer.mozilla.org/en-US/docs/Tools/Responsive_De...