

Niri’s workspaces are just rows, windows being expelled from the columns when fullscreened is very much expected.
I suppose what you want is floating the window and making it fullscreen, returning it to it’s position in the layout after returning it to it’s normal size?
To be honest, sounds like a bit of pain to implement.

Well, it’s because the whole column is fullscreened instead of a single window in that case. I guess you can abuse it a little bit and write a script that’d switch the column into tabbed mode before fullscreening a window if the column has more than one window in it.
I don’t think there is a way to know if the column is already tabbed, though. Might need a bit of unholy incantations to make it all work.