Function jump

src/components/VirtualMessageList.tsx:609–644

(i: number, wantLast: boolean)

  // Scroll to message i's top, arm scanPending. scan-effect reads fresh
  // screen next tick. wantLast: N-into-message — screenOrd = length-1.
  function jump(i: number, wantLast: boolean): void {
    const s = scrollRef.current;
    if (!s) return;
    const js = jumpState.current;
    const {
      getItemElement,
      scrollToIndex
    } = js;
    // offsets is a Float64Array whose .length is the allocated buffer (only
    // grows) — messages.length is the logical item count.
    if (i < 0 || i >= js.messages.length) return;
    // Clear stale highlight before scroll. Between now and the seek
    // effect's highlight, inverse-only from scan-highlight shows.
    setPositions?.(null);
    elementPositions.current = {
      msgIdx: -1,
      positions: []
    };
    scanRequestRef.current = {
      idx: i,
      wantLast,
      tries: 0
    };
    const el = getItemElement(i);
    const h = el?.yogaNode?.getComputedHeight() ?? 0;
    // Mounted → precise scrollTo. Unmounted → scrollToIndex mounts it
    // (scrollTop and topSpacer agree via the same offsets value — exact
    // by construction, no estimation). Seek effect does the precise
    // scrollTo after paint either way.
    if (el && h > 0) {
      s.scrollTo(targetFor(i));
    } else {
      scrollToIndex(i);
    }
    bumpSeek();
  }

  // Advance screenOrd within elementPositions. Exhausted → ptr advances,
  // jump to next matches[ptr], re-scan. Phantom (scan found 0 after