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Re:Polypane – Browser for Developers by trashburger on Saturday April 25, 2020 @03:37PM (#22980740)
BrowserSync[0] has syncing different tabs, reloading the page on code changes, layout mode, and many other features. It basically does what Polypane seems to do, and it's FOSS. The only thing missing is, as you mentioned, multiple windows side-by-side, and you can pretty much do that manually (or with a tiling WM - which you already mentioned).

[0]: https://www.browsersync.io/

Re:Polypane – Browser for Developers by dragonshed on Sunday April 26, 2020 @05:55AM (#22985951)
browsersync is nice but it's just a different approach (proxying for multiple clients/tabs) with its own tradeoffs.

Polypane is one browser tab (1 debugger, 1 css editor, etc) with multiple viewports, so there's no extra hassle or window-management involved when inspecting an element, editing a style, hitting a breakpoint, etc.