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Tabbed windows like Haiku has. I love that feature so much but I've only ever seen it on tiling WMs on Linux
I think the Forge extension for GNOME has that feature
KDE has this.
Does it? I've never been able to find it
In Dolphin, right-click and then choose "Open in New Tab".
They mean being able to group several windows together as tabs, rather than tabs implemented in specific applications.
What would that look like?
Just like a bunch of windows stacked on top of each other that move and resize together with tabs in the title bar to switch between them
Here's Haiku's explanation (this also covers the tile feature which I had forgotten about but is also super useful): https://www.haiku-os.org/docs/userguide/en/gui.html#stack-tile
I think it had at one point and was removed.
I'm looking at it right now.