feat(studio): revamps Studio + improves code quality (#2291)

* feat(studio): glue API coexistence layer for the NLE swap

What: extends 21 glue files so the OLD timeline/canvas engine and the NEW
NLE components type-check side by side: playerStore (multi-select setters,
zoom pin, snap toggle, non-reactive scale scratch), drag-state types gain
optional NLE fields, timelineLayout/timelineAssetDrop/timelineEditingHelpers/
timelineEditing/timelineElementHelpers/studioHelpers/assetHelpers gain the
NLE exports, DomEditOverlay + gestures + AssetContextMenu + Timeline props
gain optional callbacks/params, contexts gain *Optional hooks, and
TimelineEditCallbacks.onMoveElements becomes a bivariant method accepting
both engines' change shapes. patchDocumentRootDuration's test rides along.

Why: this is the keystone that dissolves the old "welded glue" problem —
every symbol the NLE components need is ADDED next to what the old engine
still uses, so the engine components and the swaps can land as separate
reviewable PRs.

How: 15 authored intermediate files (main content + additive symbols; no
behavior changes — new fields optional, new callbacks unused until wired)
plus 6 files whose final content is already purely additive. New exports
without consumers yet carry TEMP(studio-dnd) ignoreExports entries, removed
by the app-shell swap.

Test plan: tsc --noEmit in studio + studio-server (verifies BOTH engines
compile); bunx vitest run (full suite green incl. the 6 new
patchDocumentRootDuration tests); fallow audit clean.

* feat(studio): timeline interaction hooks and lanes component (unwired)

What: the timeline-side wiring layer, unwired: TimelineLanes (the lane
renderer driving drag/resize/marquee), timelineMarquee (+tests),
useTimelineStackingSync, useTimelineGeometry, useTimelineEditPinning,
useTimelineEditingDrops.

Why: everything between the pure drag math and <Timeline> itself; the
timeline-glue swap PR then only rewires Timeline/TimelineCanvas onto these.

How: new files, tsc-clean against the coexistence layer. Unwired components
carry TEMP(studio-dnd) entry registrations, removed at the app-shell swap.

Test plan: bunx vitest run timelineMarquee.test.ts; tsc --noEmit; fallow
audit clean.

* feat(studio): NLE shell assembly (unwired)

What: EditorShell (the full editor layout replacing NLELayout +
StudioPreviewArea), TimelinePane (timeline host with sub-comp rebasing) and
useTimelineEditCallbacks (the callback bag bridging store edits to the
timeline), all unwired.

Why: the shell that App swaps to in the final step; reviewing it standalone
keeps that swap PR small.

How: new files against the coexistence layer; TEMP(studio-dnd) entries
until App mounts EditorShell in the app-shell swap.

Test plan: tsc --noEmit; bunx vitest run (suite unchanged); fallow audit
clean.

* feat(studio): timeline glue swap — Timeline/TimelineCanvas onto the NLE engine

What: flips the timeline glue to its final form (23 files): Timeline and
TimelineCanvas rebuilt on TimelineLanes/TimelineOverlays, useTimelineClipDrag
drives preview/commit through the new drag engine, range selection goes
multi-select, playback loop moves to useTimelinePlayerLoop. Deletes the 9
old-engine files this orphans (group drag, marquee selection, snap targets,
layer gutter, selection overlays + their suites) — each is compile- or
gate-forced by this swap, verified by probe.

Why: second swap step; timeline-only, canvas and App untouched.

How: modified files to final content + forced deletions.
playerStore/timelineEditing/timelineCallbacks stay at their coexistence
form until the app swap (the old App still runs on them).

Test plan: tsc --noEmit; bunx vitest run (full suite); fallow audit clean.

* feat(studio): clip thumbnail modules

What: ImageThumbnail (+tests) and thumbnailUtils (+tests) — frame decode
with SVG/AVIF format fallbacks and rounded-corner clipping — plus
VideoThumbnail updates.

Why: the decode layer for timeline clip thumbnails, ahead of the visual
refresh that renders them.

How: new modules + one modified file; purely presentational.

Test plan: bunx vitest run on both test files; tsc --noEmit; fallow audit
clean.

* feat(studio): assets/blocks panel behaviors + preview helpers

What: blocks tab install flow, right-panel and global drag-overlay polish,
music beat analysis and clip-content rendering hooks, and the
preview-helper utilities backing asset preview.

Why: completes the studio NLE stack on top of the visual refresh.

How: modified files only (kept as one PR: splitting further would produce
sub-150-LOC fragments of interdependent panel glue).

Test plan: bunx vitest run studioPreviewHelpers/studioUrlState suites; tsc
--noEmit; fallow audit clean.

* fix(studio): restore timeline playback loop

* fix(studio): restore missing GSAP helpers module

* refactor(studio): split timeline GSAP helpers

* style(studio): keep timeline helper under size limit

* fix(studio): restore timeline overlays module

* fix(studio): remove stale GSAP import

* fix(studio): restore canonical timeline dependencies

* style(studio): format restored timeline helpers

* style(studio): satisfy helper line limit

* fix(studio): repair rebuilt timeline integration

* feat(studio): complete rebuilt NLE cutover

* fix(studio): guard project and timeline race boundaries

* fix(studio): preserve graded resize and crop geometry

* fix(studio): log resize/rotate commit failures, move anchor accumulator to resize-local

* fix(studio): treat duration-0 tweens as static holds and settle resize position before persist

Instant holds (to()/fromTo() with duration 0) were classified as animated
tweens by every commit route, so resizing or rotating them converted the
hold into a corrupt duration-0 keyframes tween (new value at 0%, old at
100%) that GSAP drops; panel edits appended a losing set. A shared
isInstantHold() now routes them through the static replace-in-place path,
and percentage math guards zero-duration windows.

Separately, anchored-corner resizes painted 3-5 frames at the new size but
old position while the offset persist round-tripped the server. The commit
path now applies the corrected GSAP position synchronously before awaiting
the offset persist, mirroring the scale route's settle.

* feat(studio): gesture-transaction seam with commit observability

Introduce runGestureTransaction — one owner for a gesture commit's
settle -> persist -> record lifecycle. It settles the live DOM
synchronously before any async persist, folds every mutation into one
undo entry via a per-transaction coalesceKey, restores pre-gesture state
exactly once on failure, and asserts (dev console) + reports (PostHog:
commit_transaction / commit_invariant_violation / commit_transaction_failed)
that a persist never changes pixels. The box-size resize path is migrated
onto it; the ad hoc per-route coalesceKey/reload handling is removed.

Extract the resize draft-rect math into resizeDraft.ts to keep the
gesture-handler file under the size cap.

Also: keep url_hash telemetry to the route slug only (drop the query
string, which carried the user's selected element id/selector), and gate
the [hf-resize] diagnostics behind localStorage hf-resize-debug so they
ship as opt-in tracing rather than console noise.

* fix(studio): transaction owns the undo label

The coalesced history entry took the last sub-mutation's label, so a
resize surfaced as "Move layer" (the offset persist) in undo/redo. The
seam now stamps tx.label on every wrapped mutation, so the folded entry
reads as the gesture.

* fix(studio): atomic static size/position commits (no data loss)

Static resize/position holds updated an existing set via delete+add — two
undo entries, and a delete that succeeded before a failed add lost the
hold on disk. Use one in-place update-properties mutation when a set
exists (one undo entry, no partial-failure window). The keyframed-hold
heal that can't be expressed as a property update now adds before it
deletes, so any single failure leaves a recoverable duplicate, never a
lost hold. Transaction-owned commits are tracked via a WeakSet so the
heal path never double-wraps an already-wrapped gesture.

* fix(core): restore timed-clip visibility after a forced timeline rebind

__hfForceTimelineRebind force-rendered the re-registered timeline but never
re-ran the per-[data-start] visibility pass, so after undo or soft reload
every clip rendered regardless of its time window until a full page reload.
Extract the visibility loop into syncTimedElementVisibility and call it from
both syncMediaForCurrentState (unchanged) and the rebind.

* fix(studio): atomic z-order/keyframe/split commits, one undo entry each

Three edit-commit paths hardened onto the one-transaction invariant:

- Z-order reorder (useElementLifecycleOps): N per-element writes now fold
  into one undo entry (coalesceMs Infinity) and, on a failed persist,
  restore already-written files to disk so no partial reorder survives.
- Enable-keyframes (useEnableKeyframes/useGsapKeyframeOps): the intermediate
  convert phase no longer full-reloads the preview (skipReload), killing the
  black-flash remount; convert + edit share one coalesce key = one undo entry.
- Razor split-all (useRazorSplit): snapshot before the batch and restore on
  any failure, so a mid-batch error never leaves un-revertable partial splits.

Shared file-history helpers (RecordEditInput, DomEditCommitBaseParams,
readProjectFileContent, restoreFilesToOriginal) dedupe the rollback/commit
logic across these paths. Commit options thread as one partial object rather
than field-by-field. Test setup extracted into colocated helpers.

* fix(studio): fold multi-step edits into one undo entry; guard text revert

- Gesture recording (useGestureCommit): the per-property-group commits now
  share one coalesce key and only the last reloads, so a recording is one
  undo entry and one preview reload instead of up to four.
- Delete selected keyframes (deleteSelectedKeyframes, split out of
  timelineEditingHelpers): N removals fold into one coalesced undo entry
  with a single reload.
- Text-field commit (useDomEditTextCommits): commitDomTextFields now uses
  the same version-guarded revert as handleDomTextCommit, so a stale failed
  commit can no longer stomp a newer successful one.

* feat(studio): batch a gesture's mutations into one atomic server write

A transaction that emits N mutations previously did N sequential POSTs,
each rewriting the file and soft-reloading — the root of the multi-phase
persist window. Add a gsap-mutations-batch endpoint that validates every
mutation up front, applies them in one in-memory rewrite chain, and writes
the file once (all-or-nothing: an invalid entry rejects the whole batch,
no partial write). The seam buffers a transaction's commits and, when more
than one targets the same file, dispatches a single batch — one write, one
history entry, one reload. The batch capability rides on the existing
commit-function reference; no option fields are threaded through callers.

* fix(studio): soften off-canvas indicator outline to 30% opacity

The dashed off-canvas selection outline at 60% was noisy with many
protruding elements on screen; drop the resting opacity to 30% (hover
still restores full opacity so it stays discoverable).

* fix(studio): drop off-canvas indicator outline to 10% opacity

Follow-up to the 30% softening — 10% resting opacity reads much calmer
with many protruding elements; hover still restores full opacity.

* fix(studio): gate [hf-commit] console traces to dev only

The start/settled/persisted/restore lifecycle traces logged on every
gesture commit in all environments — console noise for end users. Route
them through a dev-only traceCommit helper (matching the pixel-violation
error's existing DEV gate). The commit_* PostHog events stay always on;
they are the production observability, the console lines are a dev aid.

* fix(studio): count actual reloads, not softReload requests, in commit telemetry

A resize's size and offset persists both request softReload; the seam
counted each request, so a batched gesture reported reload_count 2 even
though the batch is one write and one reload. Compute the count from what
dispatchBufferedCommits actually did — one for a batch, the request count
for the sequential fallback.

* fix(studio): rotate hover + off-canvas overlays with the element; flicker-free crop

- Hover overlay applied the element's rotation only to the selection chrome,
  not the hover box; it now rotates about center like the selection, via a
  shared orientedGroupAwareOverlayRect router (one owner for rotation-aware
  overlay geometry across hover/selection/off-canvas).
- Off-canvas indicator was axis-aligned; it now rotates with the element and
  inverse-rotates the canvas-exclusion clip into the element's local frame,
  so the protruding-sliver clip stays correct for rotated elements.
- Crop commit re-lifted the element only in the commit's .then(), so one
  frame painted the cropped state (the flicker). Re-lift synchronously right
  after onStyleCommit (which applies the clip before its first await), so the
  cropped state never paints; the persisted file value is unchanged.

* fix(studio): address code-review findings across the commit-hardening campaign

Correctness (would ship green, bite under latency):
- Enable-keyframes phase 2 now carries coalesceMs: Infinity, so the convert
  folds into one undo entry instead of splitting past the 300ms default.
- The SDK keyframe persist path forwards coalesceMs (CutoverOptions gains the
  field); multi-keyframe delete and convert coalesce correctly when SDK-routed.
- Razor split-all's rollback is guarded so a failing restore can't swallow the
  error toast that tells the user the split failed.

Simplification (single source of truth / no dead flexibility):
- Decompose resolveResizeDraftRect (drops a fallow-ignore suppression).
- Delegate the third readProjectFileContent copy to the shared helper.
- Inline setPatchFromUpdateProperties (its only caller passes one mutation).
- One toSdkPersistOptions translates gesture overrides to SDK options.
- Bundle the reorder-rollback deps into one object (was 7-9 positional args).
- Dedupe the 'last group reloads' ternary; type gesture options as
  CommitMutationOptions; drop a Map+array wrapper around a single write.

* feat(studio): atomic z-order reorder via batch patch-element endpoint

Z-order reorder issued N per-element inline-style patches (one server
write each), so a mid-chain failure could leave a partial reorder on disk.
Add a patch-elements-batch endpoint that validates every patch, folds them
over the file in one in-memory rewrite, and writes once (all-or-nothing;
unsafe input rejects with no write). The reorder now sends one batch per
source file and records one undo entry. Because a failed atomic write
persists nothing, the interim disk-write-back rollback (restoreReorderedFile
/ restoreFulfilledReorderFiles / ReorderRollbackDeps) is deleted — failure
rolls back only live DOM/store state. Closes the last disk-atomicity gap.

* fix(studio): razor-split undo no longer silently no-ops

The split clone was written to disk without a data-hf-id, so the split
endpoint recorded that unstamped HTML as the undo entry's afterHash. The
next reloadPreview() ran the preview route's ensureHfIds write-back, which
minted a fresh id and persisted DIFFERENT bytes — so at undo time the disk
hash no longer matched afterHash and editHistory's content-mismatch guard
silently refused the undo (no write, no network, no error). Stamp the split
output via ensureHfIds in splitElementInHtml before it is written/returned,
so the preview write-back is a no-op and the recorded afterHash always
equals the final on-disk bytes. Fixes at the source rather than relaxing the
mismatch guard. Corrects the stale comment that credited forceReloadSdkSession.

* feat(studio): closed-hand grab cursor on the rotate handle

The rotate handle used the default arrow cursor; show a grabbing
(closed-hand) cursor on hover to signal it's grabbed and dragged to rotate.

* fix(studio): dropping a dragged element over another no longer selects it

A moved drag's release fired the box click, which re-selected whatever now
sat under the pointer via the hover cache — so dropping an element over a
higher-z one selected the drop target instead of keeping the dragged
element selected. The drag-move branch now suppresses the next box click,
mirroring the resize branch.

* fix(studio): group drag is one undo entry, not one per element

Dragging a multi-selected group committed each member's position write as
its own undo entry, so reverting took N Cmd+Z presses. Force a shared
coalesceKey (infinite window) across every member's commit so they fold
into a single undo entry, like the other multi-step commit paths.

* fix(studio): undo of a split no longer leaves a ghost clip in the timeline

The file and the composition iframe revert correctly on undo, but the
timeline panel kept a ghost node for the split clone. The element-merge
that repopulates the timeline preserves elements the fresh scan dropped —
intended for enriched sub-composition children a bare DOM re-scan misses,
but it also preserved a genuinely-removed TOP-LEVEL element (the split
clone after undo), leaving a phantom clip. Restrict the preserve to
elements with a compositionSrc (the enriched sub-comp children); a
top-level element missing from the fresh scan was truly removed.

---------

Co-authored-by: ukimsanov <ular.kimsanov@heygen.com>

packages/studio/src/player/hooks/useExpandedTimelineElements.ts

@@ -192,12 +192,9 @@ function domSiblingClips(
         start: host.start,
         duration: host.duration,
         track: host.track,
-        zIndex: 0,
-        stackingContextId: host.stackingContextId ?? host.domId ?? host.id ?? null,
         kind: "element",
         tagName: null,
         compositionId: null,
-        compositionAncestors: host.compositionAncestors ?? [],
         parentCompositionId: host.id ?? null,
         compositionSrc: host.compositionSrc ?? null,
         assetUrl: null,

packages/studio/src/player/hooks/usePlaybackKeyboard.ts

@@ -9,6 +9,7 @@
 import { useRef, useCallback } from "react";
 import { useCaptionStore } from "../../captions/store";
 import { shouldIgnorePlaybackShortcutEvent, SHUTTLE_SPEEDS } from "../lib/playbackShortcuts";
+import { canvasNudgeKeysClaimed } from "../../utils/canvasNudgeGate";
 import { usePlayerStore } from "../store/playerStore";
 import { stepFrameTime, STUDIO_PREVIEW_FPS } from "../lib/time";
 import type { PlaybackAdapter } from "../lib/playbackTypes";
@@ -97,6 +98,10 @@ export function usePlaybackKeyboard({
         togglePlay();
         return;
       }
+      // A nudgeable canvas selection owns the arrow keys (DomEditOverlay moves
+      // the element); frame-stepping would double-handle the same keystroke.
+      const arrowStep = e.code === "ArrowLeft" || e.code === "ArrowRight";
+      if (arrowStep && canvasNudgeKeysClaimed()) return;
       if (e.code === "ArrowLeft") {
         e.preventDefault();
         stepFrames(e.shiftKey ? -10 : -1);

packages/studio/src/player/hooks/useTimelinePlayer.ts

@@ -3,6 +3,7 @@ import { usePlayerStore, liveTime, type TimelineElement } from "../store/playerS
 import { useMountEffect } from "../../hooks/useMountEffect";
 import { usePlaybackKeyboard } from "./usePlaybackKeyboard";
 import { useTimelineSyncCallbacks } from "./useTimelineSyncCallbacks";
+import { useTimelinePlayerLoop } from "./useTimelinePlayerLoop";
 
 export type { ClipManifestClip } from "../lib/playbackTypes";
 export { createStaticSeekPlaybackAdapter } from "../lib/playbackAdapter";
@@ -15,7 +16,6 @@ export {
   parseTimelineFromDOM,
   readTimelineDurationFromDocument,
   resolveStandaloneRootCompositionSrc,
-  resolveIframe,
 } from "../lib/timelineDOM";
 export {
   shouldIgnorePlaybackShortcutEvent,
@@ -36,6 +36,7 @@ import {
   mergeTimelineElementsPreservingDowngrades,
   parseTimelineFromDOM,
 } from "../lib/timelineDOM";
+import { normalizeToZones } from "../components/timelineZones";
 import {
   setPreviewMediaMuted,
   setPreviewPlaybackRate,
@@ -90,12 +91,17 @@ export function useTimelinePlayer() {
       // applyCachedSourceDurations re-applies the cached probe duration: re-derived
       // elements (e.g. after a clip move) can arrive without sourceDuration, which
       // otherwise makes trimmed waveforms lose their window.
-      const mergedElements = applyCachedSourceDurations(
-        mergeTimelineElementsPreservingDowngrades(
-          state.elements,
-          elements,
-          state.duration,
-          resolvedDuration,
+      // Enforced CapCut zoning (overlay → main → audio): normalize track indices
+      // on every discovery. Idempotent — already-zoned input is returned as-is, so
+      // drops persist zoned indices and reloads re-zone to the same (no drift).
+      const mergedElements = normalizeToZones(
+        applyCachedSourceDurations(
+          mergeTimelineElementsPreservingDowngrades(
+            state.elements,
+            elements,
+            state.duration,
+            resolvedDuration,
+          ),
         ),
       );
 
@@ -207,49 +213,14 @@ export function useTimelinePlayer() {
     }
   }, []);
 
-  const stopReverseLoop = useCallback(() => {
-    cancelAnimationFrame(reverseRafRef.current);
-  }, []);
+  const { startRAFLoop, stopRAFLoop, stopReverseLoop } = useTimelinePlayerLoop({
+    rafRef,
+    reverseRafRef,
+    getAdapter,
+    setCurrentTime,
+    setIsPlaying,
+  });
 
-  const startRAFLoop = useCallback(() => {
-    // fallow-ignore-next-line complexity
-    const tick = () => {
-      const adapter = getAdapter();
-      if (adapter) {
-        const rawTime = adapter.getTime();
-        const dur = adapter.getDuration();
-        const time = dur > 0 ? Math.min(rawTime, dur) : rawTime;
-        liveTime.notify(time); // direct DOM updates, no React re-render
-        const { inPoint, outPoint } = usePlayerStore.getState();
-        const rawLoopEnd = outPoint !== null ? Math.min(outPoint, dur) : dur;
-        const rawLoopStart = inPoint !== null ? inPoint : 0;
-        const loopEnd = rawLoopStart < rawLoopEnd ? rawLoopEnd : dur;
-        const loopStart = rawLoopStart < rawLoopEnd ? rawLoopStart : 0;
-        if (time >= loopEnd) {
-          if (usePlayerStore.getState().loopEnabled && dur > 0) {
-            // keepPlaying skips the adapter's implicit pause; play() below is then a no-op.
-            adapter.seek(loopStart, { keepPlaying: true });
-            liveTime.notify(loopStart);
-            adapter.play();
-            setIsPlaying(true);
-            rafRef.current = requestAnimationFrame(tick);
-            return;
-          }
-          if (adapter.isPlaying()) adapter.pause();
-          setCurrentTime(time); // sync Zustand once at end
-          setIsPlaying(false);
-          cancelAnimationFrame(rafRef.current);
-          return;
-        }
-      }
-      rafRef.current = requestAnimationFrame(tick);
-    };
-    rafRef.current = requestAnimationFrame(tick);
-  }, [getAdapter, setCurrentTime, setIsPlaying]);
-
-  const stopRAFLoop = useCallback(() => {
-    cancelAnimationFrame(rafRef.current);
-  }, []);
   const applyPlaybackRate = useCallback((rate: number) => {
     const iframe = iframeRef.current;
     if (!iframe) return;
@@ -463,10 +434,24 @@ export function useTimelinePlayer() {
       applyPreviewAudioState,
     });
   const saveSeekPosition = useCallback(() => {
-    const adapter = getAdapter();
-    pendingSeekRef.current = adapter
-      ? adapter.getTime()
-      : (usePlayerStore.getState().currentTime ?? 0);
+    // Never DEGRADE the saved position. Overlapping reloads (e.g. an external
+    // file drop = upload reload + insert reload back-to-back) call this while
+    // the iframe from the FIRST reload is mid-teardown: getAdapter() can still
+    // return that dying document's adapter, whose getTime() reads 0 — and the
+    // store's currentTime can lag the visual playhead. Overwriting the
+    // still-unconsumed pendingSeek with either value is exactly how the
+    // playhead used to end up at 0 after a Finder drop (verified live via a
+    // currentTime write-trace). So: while a refresh is already in flight and a
+    // save exists, keep it; otherwise trust the live adapter, then the store.
+    const refreshInFlight = isRefreshingRef.current && pendingSeekRef.current != null;
+    if (!refreshInFlight) {
+      const adapter = getAdapter();
+      if (adapter) {
+        pendingSeekRef.current = adapter.getTime();
+      } else if (pendingSeekRef.current == null) {
+        pendingSeekRef.current = usePlayerStore.getState().currentTime ?? 0;
+      }
+    }
     isRefreshingRef.current = true;
     stopRAFLoop();
     stopReverseLoop();
@@ -476,6 +461,16 @@ export function useTimelinePlayer() {
     const iframe = iframeRef.current;
     if (!iframe) return;
     saveSeekPosition();
+    // Hide the iframe across the full reload so the user never sees the reloading
+    // document's RAW DOM (every clip stacked and visible) in the window between the
+    // new document parsing and the runtime initializing + seeking. initializeAdapter
+    // reveals it again right after its restore seek renders the correct frame.
+    // Tradeoff: this shows the parent stage background (a brief "freeze"/blank, on
+    // the order of the reload time ~100-300ms) INSTEAD of the all-clips flash. A
+    // blank is far less jarring than a burst of every asset appearing at once.
+    // Only the FULL-reload edits (drops/inserts) hit this — timing edits now take
+    // the soft-reload path and never touch refreshPlayer.
+    iframe.style.visibility = "hidden";
     const src = iframe.src;
     const url = new URL(src, window.location.origin);
     url.searchParams.set("_t", String(Date.now()));

packages/studio/src/player/hooks/useTimelinePlayerLoop.ts

@@ -0,0 +1,85 @@
+/**
+ * The forward playback loop for the timeline player.
+ *
+ * Owns the three requestAnimationFrame lifecycle callbacks that drive (and stop)
+ * playback:
+ *  - startRAFLoop     — the forward tick: advance liveTime, honour in/out loop
+ *                       points + loopEnabled, and pause + sync the store at the end.
+ *  - stopRAFLoop      — cancel the forward tick.
+ *  - stopReverseLoop  — cancel the reverse-shuttle tick (owned by the parent hook).
+ *
+ * Called unconditionally at the top level of useTimelinePlayer so its useCallback
+ * hooks run in a stable order; every dependency is passed in as an argument.
+ */
+
+import { useCallback } from "react";
+import { liveTime, usePlayerStore } from "../store/playerStore";
+import type { PlaybackAdapter } from "../lib/playbackTypes";
+
+interface UseTimelinePlayerLoopParams {
+  rafRef: React.MutableRefObject<number>;
+  reverseRafRef: React.MutableRefObject<number>;
+  getAdapter: () => PlaybackAdapter | null;
+  setCurrentTime: (v: number) => void;
+  setIsPlaying: (v: boolean) => void;
+}
+
+interface UseTimelinePlayerLoopResult {
+  startRAFLoop: () => void;
+  stopRAFLoop: () => void;
+  stopReverseLoop: () => void;
+}
+
+export function useTimelinePlayerLoop({
+  rafRef,
+  reverseRafRef,
+  getAdapter,
+  setCurrentTime,
+  setIsPlaying,
+}: UseTimelinePlayerLoopParams): UseTimelinePlayerLoopResult {
+  const stopReverseLoop = useCallback(() => {
+    cancelAnimationFrame(reverseRafRef.current);
+  }, [reverseRafRef]);
+
+  const startRAFLoop = useCallback(() => {
+    // fallow-ignore-next-line complexity
+    const tick = () => {
+      const adapter = getAdapter();
+      if (adapter) {
+        const rawTime = adapter.getTime();
+        const dur = adapter.getDuration();
+        const time = dur > 0 ? Math.min(rawTime, dur) : rawTime;
+        liveTime.notify(time); // direct DOM updates, no React re-render
+        const { inPoint, outPoint } = usePlayerStore.getState();
+        const rawLoopEnd = outPoint !== null ? Math.min(outPoint, dur) : dur;
+        const rawLoopStart = inPoint !== null ? inPoint : 0;
+        const loopEnd = rawLoopStart < rawLoopEnd ? rawLoopEnd : dur;
+        const loopStart = rawLoopStart < rawLoopEnd ? rawLoopStart : 0;
+        if (time >= loopEnd) {
+          if (usePlayerStore.getState().loopEnabled && dur > 0) {
+            // keepPlaying skips the adapter's implicit pause; play() below is then a no-op.
+            adapter.seek(loopStart, { keepPlaying: true });
+            liveTime.notify(loopStart);
+            adapter.play();
+            setIsPlaying(true);
+            rafRef.current = requestAnimationFrame(tick);
+            return;
+          }
+          if (adapter.isPlaying()) adapter.pause();
+          setCurrentTime(time); // sync Zustand once at end
+          setIsPlaying(false);
+          cancelAnimationFrame(rafRef.current);
+          return;
+        }
+      }
+      rafRef.current = requestAnimationFrame(tick);
+    };
+    rafRef.current = requestAnimationFrame(tick);
+  }, [rafRef, getAdapter, setCurrentTime, setIsPlaying]);
+
+  const stopRAFLoop = useCallback(() => {
+    cancelAnimationFrame(rafRef.current);
+  }, [rafRef]);
+
+  return { startRAFLoop, stopRAFLoop, stopReverseLoop };
+}