[skgough]

One feature of i3 and friends that I really relied on when I was using a laptop as my main computer was the tab mode. Being able to tab between windows on half of your screen while keeping your browser open in the other half was extremely useful.

I know BeOS had tabbed windows in the 90s in a floating window manager; it makes me wonder why this idea didn't catch on in the early 2000s.

Windows has started to add tabs to individual programs incrementally as part of their rewrites of core applications in the new GUI frameworks. Notepad got tabs and so did Explorer. So they see the utility.

Why hasn't tabbing been included as a core feature of the window manager outside of these niche tiling window managers for Linux?

[solomonb]

You can do tabbed windows in XMonad.