Tabs should be a part of the window manager, not the application. This is a hill i have been on for a long time (before i changed the direction of celeste, it was one of the things that was around for the longest). There is no reason why all things shouldn't be able to be tabbed together rather than segregated by application.
There are a few instances of this around. Haiku gets mentioned a lot, but partly i think due to the design happening before tabs were really a big thing, the implementation isn't great due to a lack of quality of life features, like being easy to use.
A few linux window managers also have tabs at the window manager level, but i have also not managed to get these to work very successfully.
Chrome os was the time when i properly realised the potential, although it is only around in a limited form. But because everything is in chrome, most things work quite well with one tab per application.
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