Function useVirtualScroll

src/hooks/useVirtualScroll.ts:142–721

(
  scrollRef: RefObject<ScrollBoxHandle | null>,
  itemKeys: readonly string[],
  /**
   * Terminal column count. On change, cached heights are stale (text
   * rewraps) — SCALED by oldCols/newCols rather than cleared. Clearing
   * made the pessimistic coverage back-walk mount ~190 items (every
   * uncached item → PESSIMISTIC_HEIGHT=1 → walk 190 to reach
   * viewport+2×overscan). Each fresh mount runs marked.lexer + syntax
   * highlighting ≈ 3ms; ~600ms React reconcile on first resize with a
   * long conversation. Scaling keeps heightCache populated → back-walk
   * uses real-ish heights → mount range stays tight. Scaled estimates
   * are overwritten by real Yoga heights on next useLayoutEffect.
   *
   * Scaled heights are close enough that the black-screen-on-widen bug
   * (inflated pre-resize offsets overshoot post-resize scrollTop → end
   * loop stops short of tail) doesn't trigger: ratio<1 on widen scales
   * heights DOWN, keeping offsets roughly aligned with post-resize Yoga.
   */
  columns: number,
)

 * the last N items regardless of what scrollTop claims.
 */
export function useVirtualScroll(
  scrollRef: RefObject<ScrollBoxHandle | null>,
  itemKeys: readonly string[],
  /**
   * Terminal column count. On change, cached heights are stale (text
   * rewraps) — SCALED by oldCols/newCols rather than cleared. Clearing
   * made the pessimistic coverage back-walk mount ~190 items (every
   * uncached item → PESSIMISTIC_HEIGHT=1 → walk 190 to reach
   * viewport+2×overscan). Each fresh mount runs marked.lexer + syntax
   * highlighting ≈ 3ms; ~600ms React reconcile on first resize with a
   * long conversation. Scaling keeps heightCache populated → back-walk
   * uses real-ish heights → mount range stays tight. Scaled estimates
   * are overwritten by real Yoga heights on next useLayoutEffect.
   *
   * Scaled heights are close enough that the black-screen-on-widen bug
   * (inflated pre-resize offsets overshoot post-resize scrollTop → end
   * loop stops short of tail) doesn't trigger: ratio<1 on widen scales
   * heights DOWN, keeping offsets roughly aligned with post-resize Yoga.
   */
  columns: number,
): VirtualScrollResult {
  const heightCache = useRef(new Map<string, number>())
  // Bump whenever heightCache mutates so offsets rebuild on next read. Ref
  // (not state) — checked during render phase, zero extra commits.
  const offsetVersionRef = useRef(0)
  // scrollTop at last commit, for detecting fast-scroll mode (slide cap gate).
  const lastScrollTopRef = useRef(0)
  const offsetsRef = useRef<{ arr: Float64Array; version: number; n: number }>({
    arr: new Float64Array(0),
    version: -1,
    n: -1,
  })
  const itemRefs = useRef(new Map<string, DOMElement>())
  const refCache = useRef(new Map<string, (el: DOMElement | null) => void>())
  // Inline ref-compare: must run before offsets is computed below. The
  // skip-flag guards useLayoutEffect from re-populating heightCache with
  // PRE-resize Yoga heights (useLayoutEffect reads Yoga from the frame
  // BEFORE this render's calculateLayout — the one that had the old width).
  // Next render's useLayoutEffect reads post-resize Yoga → correct.
  const prevColumns = useRef(columns)
  const skipMeasurementRef = useRef(false)
  // Freeze the mount range for the resize-settling cycle. Already-mounted
  // items have warm useMemo (marked.lexer, highlighting); recomputing range
  // from scaled/pessimistic estimates causes mount/unmount churn (~3ms per
  // fresh mount = ~150ms visible as a second flash). The pre-resize range is
  // as good as any — items visible at old width are what the user wants at
  // new width. Frozen for 2 renders: render #1 has skipMeasurement (Yoga
  // still pre-resize), render #2's useLayoutEffect reads post-resize Yoga
  // into heightCache. Render #3 has accurate heights → normal recompute.
  const prevRangeRef = useRef<readonly [number, number] | null>(null)
  const freezeRendersRef = useRef(0)
  if (prevColumns.current !== columns) {
    const ratio = prevColumns.current / columns
    prevColumns.current = columns
    for (const [k, h] of heightCache.current) {
      heightCache.current.set(k, Math.max(1, Math.round(h * ratio)))
    }
    offsetVersionRef.current++