That principle is often misapplied, and I think that's true here, too.
The "do one thing" about Unix is really about composability (e.g. "find" doesn't need to sort because you do "find | sort"), but you don't compose a terminal app with anything.
A terminal app that has terminal features doesn't violate any principles of simplicity.
It's rarely a significant problem in practice, but it annoys me in principle!
If you do "blah | sort", then "sort" could ask its upstream processing node whether it supported sorting on the requisite fields, and "push down" the necessary sort-order descriptor into the "blah" step.
That requires two things: That the pipe API sets up a communications channel between the two programs in a way that makes them aware of each other and able to exchange information; and secondly, that the pipe protocol is based on typed, structured data. I want both things.
Imagine if you had that, then you could conceivably also do:
psql -c "select firstname, lastname from foo" |
sort -f lastname
select firstname, lastname from foo order by lastname
The inability to do this sort of thing really a product of a failure to modernize the 1970s text-oriented pipe data model. I believe PowerShell (which I've never used, only read about) provides a mechanism to accomplish this sort of thing, at the expense of being extremely Microsoft-flavoured.
I don't think there's anything even vaguely scifi about those abilities, but the Unix world is hampered by a curious reticence to innovate certain core technologies such as, well, Unix itself. That's why we still have tmux and such.
Everything's highly composable, e.g. I can switch out bash for zsh, fish, etc. I can switch out dtach for abduco. I can switch out dvtm for tmux or screen. I can switch out st for xterm or urxvt. And so on.
Adding an extra layer for scrollback, separate from a multiplexer, wouldn't disrupt anything, and would provide more flexibility for composition.
I shaved off my neckbeard, I'll have you know! ;)
My setup's no more outlandish than using tmux or screen, except instead of typing `tmux` or `screen` I type `shell`, which aliases a `dtach dvtm` one-liner (with a few options sprinkled around, so I don't have to bother with config files).
The point is that none of these applications care if/how they're composed; if I want to add in or swap out something, it's just a change that one-liner.
Not so if, say, my terminal application were hard-coded to rely on tmux, as some sibling comments have suggested.
In AmigaOS it was fairly common for applications to reuse the standard console handler - the same one used for the shell - to provide a custom console or even editor windows etc.. For minimal integration all it takes is to read from/write to a filehandle.
That said, though as I noted elsewhere, even AmigaOS got scrollback in the console handler by '87 - the extra code measures a few KB; it'd be more hassle than it was worth to split it out.
It still depends how these features are implemented.
For example in dvtm scroll back history is made searchable by piping it to $PAGER. Similarly, copy mode is implemented by using $EDITOR as an interactive filter.