Can I ignore flatpak indefinitely?

Sure, at least until software you want to use is flatpak only, e.g. Bottles

Okay, let’s go with xterm running bash, where the user ran ls, so xterm -> bash -> ls …

• ls never talks to xterm directly, it’s stdout/stderr are provided by bash
• bash effectively outputs a grid of characters to xterm, xterm doesn’t know about prompts or words or line feeds, just the grid
• every time ls outputs a line, bash adds a row of output to the grid that it sends to xterm
• if there’s not enough space for a new row, bash discards the top-most row, moves all other rows up by one row, and then inserts the row for the ls output

Now imagine a hypothetical fork of bash or some other new shell …

• the only thing different is the direction that the rows move off the edge of the screen when running out of space, that’s all

Thus, this is entirely a shell problem, with a shell solution

However, what I’ve neglected to mention so far is that terminal emulators and shells are almost certainly optimised for rows dropping off the top edge and new rows being added to the bottom edge

So, the role of a terminal emulator in this scenario could be to provide ANSI control characters or other protocol for operating just as quickly in the opposite direction, sure

There’s also https://www.waveterm.dev/ which seems to be an open-source attempt at something sort of like Warp/Jupyter

I don’t mind that it uses the web stack for rendering, but that’ll probably turn some folks off

Seems like a shell feature, and not a feature that a terminal emulator would implement

One example I can think of is Widevine DRM, which is owned by Google and is closed source: https://en.wikipedia.org/wiki/Widevine

Google currently allows Mozilla (and others) to distribute this within Firefox, allowing Netflix, Disney+, and various other video streaming services to work within Firefox without any technical work performed by the user

I don’t believe Google would ever willingly take this away from Mozilla, but it’s entirely possible that the movie and music industries pressure Google to reduce access to Widevine (the same way they pressured Netflix into adopting DRM)