Does self-driven mode work inside a scroll container other than the window?

[aron-intframe]

The component runs its own requestAnimationFrame loop and reads scroll position with getBoundingClientRect. If I mount it inside a scroll container that is not the window (a modal with overflow: auto, for example), does the progress still track correctly, or do I have to drive it myself?

I would rather not fork the component just to swap the scroll root.

[aron-intframe]

No, self-driven mode is tied to the window viewport, so you need controlled mode there.

The whole of self-driven progress is these three lines in src/ThermalReceipt.tsx:

const rect = sec.getBoundingClientRect();
const track = rect.height - innerHeight;
progressRef.current = track > 0 ? clamp01(-rect.top / track) : 0;

getBoundingClientRect().top is measured against the viewport, not against the nearest scrolling ancestor, and innerHeight is the window height. Inside a modal with overflow: auto both are wrong for you: rect.top barely moves while the modal scrolls, and track is computed against a viewport that is taller than the box the receipt actually lives in. If the modal happens to be full-screen and scrolls the document itself, it works by accident.

Use controlled mode instead. Pass progress and drive it from your own container:

const box = useRef(null);
const [p, setP] = useState(0);

useEffect(() => {
  const el = box.current;
  const onScroll = () => {
    const track = el.scrollHeight - el.clientHeight;
    setP(track > 0 ? Math.min(1, Math.max(0, el.scrollTop / track)) : 0);
  };
  onScroll();
  el.addEventListener('scroll', onScroll, { passive: true });
  return () => el.removeEventListener('scroll', onScroll);
}, []);

<div ref={box} style={{ overflow: 'auto' }}>
  <ThermalReceipt progress={p} lines={lines} />
</div>

No fork needed. When progress is supplied the component skips the getBoundingClientRect branch entirely and only smooths the value you hand it, so every other beat (develop, feed steps, stamp, tear-off) behaves exactly as in self-driven mode.