I think we should implement things from scratch, yes.

Functionality would be quite similar to Rich's Progress. Although simpler. It only needs to render a single progress, with percentage and optional ETA.

@willmcgugan I think things are going in a pretty good direction as of 8f2fae5.

But I have an issue I don't understand.
I don't know why I need this line:

from textual.app import App, ComposeResult
from textual.widgets import ProgressBar, Footer


class MyApp(App[None]):
    BINDINGS = [
        ("a", "advance", "Advance by 1"),
        ("j", "jump", "Jump to 25"),
        ("u", "update", "Update to start"),
        ("d", "duplicate", "Duplicate total"),
    ]

    def compose(self) -> ComposeResult:
        self.pb = ProgressBar(total=None)
        yield self.pb
        yield Footer()

    def action_update(self):
        self.pb.update(total=100)

    def action_advance(self):
        self.pb.advance()

    def action_jump(self):
        self.pb.progress = 25

    def action_duplicate(self):
        self.pb.total *= 2


if __name__ == "__main__":
    MyApp().run()

Demo of the app with the progress bar:

Screenshot of a couple of progress bars (left and upper-left corner are indeterminate progress bars; not really on-brand for the app shown, but I tweaked the example just to get an indeterminate bar to show quickly.)

The video demo above is current, up to the colour of the progress bar (which can be styled with CSS either way).

I still need to add tests, but I'll wait for @willmcgugan to determine what features he'd like to see/change before testing everything.

Updated recording of the indeterminate effect with 15 FPS: