The important question becomes can you stack the window decoration "tabs" of different apps into a single stack of tabs like in BeOS?

Demonstrated here (animated):

https://www.haiku-os.org/docs/userguide/en/images/gui-images...

I used to run fluxbox in the early 2000s. I greatly miss tabbing any windows like that.

You can run fluxbox today! I still do, I can’t go without window tabs. :^)

I believe the Cosmic Desktop from Pop OS has that again

There's no equivalent on Wayland?

Sway.

me too! fluxbox and gkrellm for some kick ass desktop "widgets" monitoring the computer :D

KDE had it. And I missed it a lot.

This is what we needed in our OSes instead of Firefox tabs.

How is it that different?

basically every app is a tab. this is how I run i3wm. full screen tabbed layout. smaller modal windows still appear in their normal smaller windows in front of the current full screen app.

I tried to install a quick VM to answer this exact question but I had some difficult getting it running in Gnome Boxes.

Yes the UX is virtually the same