Function highlight

src/components/VirtualMessageList.tsx:481–525

(ord: number)

  // scrollTop fresh. If ord's position is off-viewport, scroll to bring
  // it in, recompute rowOffset. setPositions triggers overlay write.
  function highlight(ord: number): void {
    const s = scrollRef.current;
    const {
      msgIdx,
      positions
    } = elementPositions.current;
    if (!s || positions.length === 0 || msgIdx < 0) {
      setPositions?.(null);
      return;
    }
    const idx = Math.max(0, Math.min(ord, positions.length - 1));
    const p = positions[idx]!;
    const top = jumpState.current.getItemTop(msgIdx);
    // lo = item's position within scroll content (wrapper-relative).
    // viewportTop = where the scroll content starts on SCREEN (after
    // ScrollBox padding/border + any chrome above). Highlight writes to
    // screen-absolute, so rowOffset = viewportTop + lo. Observed: off-by-
    // 1+ without viewportTop (FullscreenLayout has paddingTop=1 on the
    // ScrollBox, plus any header above).
    const vpTop = s.getViewportTop();
    let lo = top - s.getScrollTop();
    const vp = s.getViewportHeight();
    let screenRow = vpTop + lo + p.row;
    // Off viewport → scroll to bring it in (HEADROOM from top).
    // scrollTo commits sync; read-back after gives fresh lo.
    if (screenRow < vpTop || screenRow >= vpTop + vp) {
      s.scrollTo(Math.max(0, top + p.row - HEADROOM));
      lo = top - s.getScrollTop();
      screenRow = vpTop + lo + p.row;
    }
    setPositions?.({
      positions,
      rowOffset: vpTop + lo,
      currentIdx: idx
    });
    // Badge: global current = sum of occurrences before this msg + ord+1.
    // prefixSum[ptr] is engine-counted (indexOf on extractSearchText);
    // may drift from render-count for ghost messages but close enough —
    // badge is a rough location hint, not a proof.
    const st = searchState.current;
    const total = st.prefixSum.at(-1) ?? 0;
    const current = (st.prefixSum[st.ptr] ?? 0) + idx + 1;
    onSearchMatchesChange?.(total, current);
    logForDebugging(`highlight(i=${msgIdx}, ord=${idx}/${positions.length}): ` + `pos={row:${p.row},col:${p.col}} lo=${lo} screenRow=${screenRow} ` + `badge=${current}/${total}`);
  }
  highlightRef.current = highlight;

  // Seek effect. jump() sets scanRequestRef + scrollToIndex + bump.