feat(studio): timeline collision and placement model (#2279)

* feat(studio): timeline collision and placement model

What: new pure module timelineCollision — zone-aware drop placement
(clampTrackToZone, resolveZoneDropPlacement, resolveInsertRow,
resolvePlacement, lane/overlap predicates) with its full test suite.

Why: the no-overlap core of the NLE clip-drag engine; plain functions, no
DOM, no React, no store writes.

How: new files only; type-only imports from the existing playerStore.
First runtime consumer arrives with the drag-engine PRs.

Test plan: bunx vitest run timelineCollision.test.ts; tsc --noEmit; fallow
audit clean (all exports test-consumed).

* feat(studio): timeline magnetic snapping

What: new pure module timelineSnapping — snap-target collection and
pixel-threshold time snapping (collectTimelineSnapTargets, snapTimelineTime,
snapMoveToTargets) with tests.

Why: the magnet math for clip drags/trims, reviewable standalone.

How: new files only; type-only playerStore imports; consumers land with the
drag engine.

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

* feat(studio): multi-clip drag preview math

What: new pure module timelineMultiDragPreview — group-drag passenger
offsets and clamped group deltas (isMultiDragActive, multiDragDeltaSeconds,
multiDragPassengerOffsetPx, clampGroupMoveDelta) with tests.

Why: the group-drag math, standalone and DOM-free.

How: new files only; consumed later by TimelineLanes.

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

* feat(studio): timeline z-stacking sync model

What: new pure module timelineStackingSync — lane order ↔ z-index
reconciliation (laneIsAbove, computeStackingPatches) with tests.

Why: the single source of truth for how timeline lane order maps to canvas
stacking; the ordering rules and tie-breaks live here.

How: new files only; consumed later by timelineZones and the stacking-sync
hook.

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

* feat(studio): timeline lane-zone model

What: new pure module timelineZones — visual/audio track-zone
classification (classifyZone) and normalizeToZones, which re-packs lanes
into zone-consistent rows; tests cover the stacking/zones interaction.

Why: completes the z-model started in the stacking-sync PR.

How: new files; consumes isAudioTimelineElement (leaf-helpers PR) and
computeStackingPatches (stacking-sync PR); type-only playerStore imports.

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

---------

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

packages/studio/src/player/components/timelineZones.test.ts

@@ -0,0 +1,425 @@
+import { describe, expect, it } from "vitest";
+import type { TimelineElement } from "../store/playerStore";
+import { classifyZone, normalizeToZones } from "./timelineZones";
+import { computeStackingPatches, type StackingElement } from "./timelineStackingSync";
+
+function el(id: string, tag: string, track: number, duration = 2): TimelineElement {
+  return { id, tag, start: 0, duration, track };
+}
+
+function zClip(
+  id: string,
+  start: number,
+  duration: number,
+  track: number,
+  zIndex: number,
+  tag = "video",
+): TimelineElement {
+  return { id, tag, start, duration, track, zIndex };
+}
+
+function trackOf(els: TimelineElement[], id: string): number {
+  return els.find((e) => e.id === id)!.track;
+}
+
+/** Assert normalizeToZones is idempotent: re-zoning keeps every clip's lane. */
+function expectZoningIdempotent(input: TimelineElement[]): void {
+  const once = normalizeToZones(input);
+  const twice = normalizeToZones(once);
+  for (const e of once) expect(trackOf(twice, e.id)).toBe(e.track);
+}
+
+/** The exact qa-clean live repro (array order = DOM order); fresh objects per call. */
+function qaCleanRepro(): TimelineElement[] {
+  return [
+    zClip("blue-logo", 6.37, 3, 0, 3, "img"),
+    zClip("ralu", 6.37, 3, 0, 0, "img"),
+    zClip("black-logo", 11.92, 3, 1, 1, "img"),
+    zClip("video", 0.84, 20, 3, 2, "video"),
+  ];
+}
+
+describe("classifyZone", () => {
+  it("audio → audio; video / image / everything else → visual", () => {
+    expect(classifyZone(el("m", "audio", 3))).toBe("audio");
+    expect(classifyZone(el("v", "video", 1))).toBe("visual");
+    expect(classifyZone(el("i", "img", 0))).toBe("visual");
+  });
+
+  it("zone identity invariant: normalizeToZones preserves each clip's zone (mixed input)", () => {
+    // normalizeToZones only remaps lanes — it must never reclassify a clip's zone.
+    const input = [
+      el("v", "video", 0),
+      el("a1", "audio", 1),
+      el("i", "img", 2),
+      el("a2", "audio", 3),
+    ];
+    const out = normalizeToZones(input);
+    for (const e of input) {
+      expect(classifyZone(out.find((o) => o.id === e.id)!)).toBe(classifyZone(e));
+    }
+    // And the partition holds: every visual lane sits above every audio lane.
+    const laneOf = (id: string) => trackOf(out, id);
+    const maxVisual = Math.max(laneOf("v"), laneOf("i"));
+    const minAudio = Math.min(laneOf("a1"), laneOf("a2"));
+    expect(maxVisual).toBeLessThan(minAudio);
+  });
+});
+
+describe("normalizeToZones", () => {
+  it("orders visual (top) → audio (bottom); equal-z overlap stacks by DOM order", () => {
+    // img and vid are both z=0, start=0 → they OVERLAP in time. CSS paints
+    // equal-z siblings by DOM order (later paints on top), so the later-in-array
+    // `vid` must own the upper lane. (Was: pinned img=0/vid=1 to authored order,
+    // which contradicts the canvas — updated for per-clip DOM-order tie-break.)
+    const out = normalizeToZones([
+      el("img", "img", 0),
+      el("vid", "video", 2),
+      el("mus", "audio", 5),
+    ]);
+    expect(trackOf(out, "vid")).toBe(0); // later in DOM → paints on top → upper lane
+    expect(trackOf(out, "img")).toBe(1); // earlier in DOM → below
+    expect(trackOf(out, "mus")).toBe(2); // audio (bottom)
+  });
+
+  it("keeps all visual lanes together on top; equal-z overlap stacks by DOM order", () => {
+    // All three z=0, start=0 → mutually overlapping. DOM order (later on top)
+    // decides the stack: v3 (last) top, then i1, then v0. (Was: pinned to authored
+    // order v0=0/i1=1/v3=2, which the canvas contradicts for overlapping equal-z.)
+    const out = normalizeToZones([el("v0", "video", 0), el("i1", "img", 1), el("v3", "video", 3)]);
+    expect(trackOf(out, "v3")).toBe(0);
+    expect(trackOf(out, "i1")).toBe(1);
+    expect(trackOf(out, "v0")).toBe(2);
+  });
+
+  it("drops audio below the visual lanes even when sharing a track index", () => {
+    const out = normalizeToZones([el("v", "video", 0), el("a", "audio", 0)]);
+    expect(trackOf(out, "v")).toBe(0); // visual
+    expect(trackOf(out, "a")).toBe(1); // audio, below
+  });
+
+  it("groups multiple audio tracks at the bottom preserving relative order", () => {
+    const out = normalizeToZones([el("v", "video", 0), el("a1", "audio", 1), el("a2", "audio", 4)]);
+    expect(trackOf(out, "v")).toBe(0);
+    expect(trackOf(out, "a1")).toBe(1);
+    expect(trackOf(out, "a2")).toBe(2);
+  });
+
+  it("returns the same array (identity) when already zoned", () => {
+    // A fixed-point layout: the two overlapping equal-z visual clips are already
+    // in DOM-order-consistent lanes (later-in-array `v` on the upper lane 0), so
+    // re-zoning is a no-op and the SAME array reference comes back. (Was: i=0/v=1,
+    // which the new per-clip DOM-order pack would flip — so it wasn't a fixed
+    // point under the corrected canvas semantics; swapped to the stable order.)
+    const input = [el("v", "video", 1), el("i", "img", 0), el("a", "audio", 2)];
+    expect(normalizeToZones(input)).toBe(input);
+  });
+
+  it("is idempotent (no drift on re-zoning)", () => {
+    const input = [
+      el("img", "img", 1),
+      el("v", "video", 3),
+      el("a1", "audio", 2),
+      el("a2", "audio", 6),
+    ];
+    expectZoningIdempotent(input);
+  });
+
+  it("splits time-overlapping clips on one track onto separate lanes (no visible overlap)", () => {
+    const clip = (id: string, start: number, duration: number): TimelineElement => ({
+      id,
+      tag: "video",
+      start,
+      duration,
+      track: 1, // all authored on the SAME track, some overlapping in time
+    });
+    // a [0,5), b [2,7) overlaps a, c [6,9) fits after a. All equal-z (absent).
+    // Per-clip pack (DOM-order tie-break): c is last in DOM so it places on the
+    // top lane 0; b overlaps c and lands on lane 1; a overlaps b (and is earlier
+    // in DOM than b, so it must paint BELOW b) → lane 2. a can NOT drop onto c's
+    // lane 0 even though it doesn't overlap c, because that would place a ABOVE b
+    // in lane order while a paints below b — the canvas-correct constraint the old
+    // whole-track packer ignored (it shared a/c on lane 0, contradicting paint).
+    const out = normalizeToZones([clip("a", 0, 5), clip("b", 2, 5), clip("c", 6, 3)]);
+    expect(trackOf(out, "c")).toBe(0); // last in DOM → top lane
+    expect(trackOf(out, "b")).toBe(1); // overlaps c → below it
+    expect(trackOf(out, "a")).toBe(2); // paints below b (earlier DOM, equal z) → lane below b
+
+    // No two time-overlapping clips share a lane (the real NLE invariant).
+    expect(trackOf(out, "a")).not.toBe(trackOf(out, "b"));
+    expect(trackOf(out, "b")).not.toBe(trackOf(out, "c"));
+
+    // Idempotent: re-laying the split result changes nothing.
+    const twice = normalizeToZones(out);
+    for (const e of out) expect(trackOf(twice, e.id)).toBe(e.track);
+  });
+});
+
+describe("normalizeToZones — reverse z→lane mapping", () => {
+  it("orders overlapping same-zone clips by z: higher z → higher (upper) lane", () => {
+    // lo (z=1) and hi (z=9) fully overlap in time on the same authored track.
+    const out = normalizeToZones([zClip("lo", 0, 10, 0, 1), zClip("hi", 0, 10, 0, 9)]);
+    expect(trackOf(out, "hi")).toBe(0); // higher z → upper lane (top)
+    expect(trackOf(out, "lo")).toBe(1); // lower z → below
+  });
+
+  it("orders three overlapping clips strictly by descending z", () => {
+    const out = normalizeToZones([
+      zClip("mid", 0, 10, 0, 5),
+      zClip("top", 0, 10, 0, 8),
+      zClip("bot", 0, 10, 0, 2),
+    ]);
+    expect(trackOf(out, "top")).toBe(0);
+    expect(trackOf(out, "mid")).toBe(1);
+    expect(trackOf(out, "bot")).toBe(2);
+  });
+
+  it("does NOT reorder non-overlapping (sequential) clips by z — they share a lane", () => {
+    // a [0,5) z=1 then c [6,9) z=9 — no time overlap, so z is irrelevant.
+    const out = normalizeToZones([zClip("a", 0, 5, 0, 1), zClip("c", 6, 3, 0, 9)]);
+    expect(trackOf(out, "a")).toBe(0);
+    expect(trackOf(out, "c")).toBe(0); // shares the lane regardless of higher z
+  });
+
+  it("leaves the audio zone unaffected by z", () => {
+    const out = normalizeToZones([
+      zClip("v", 0, 10, 0, 1),
+      zClip("m1", 0, 10, 1, 99, "audio"),
+      zClip("m2", 0, 10, 1, 0, "audio"),
+    ]);
+    // Two overlapping audio clips split onto lanes below the visual clip; their
+    // relative z does not lift one above a visual clip.
+    expect(trackOf(out, "v")).toBe(0);
+    expect(trackOf(out, "m1")).toBeGreaterThan(trackOf(out, "v"));
+    expect(trackOf(out, "m2")).toBeGreaterThan(trackOf(out, "v"));
+  });
+
+  it("treats missing / auto z as 0 (undefined z clip sinks below a positive-z overlap)", () => {
+    const out = normalizeToZones([
+      { id: "noz", tag: "video", start: 0, duration: 10, track: 0 }, // no zIndex
+      zClip("pos", 0, 10, 0, 3),
+    ]);
+    expect(trackOf(out, "pos")).toBe(0); // z=3 → upper
+    expect(trackOf(out, "noz")).toBe(1); // absent z ⇒ 0 → below
+  });
+
+  it("tie-breaks equal-z overlapping clips on the STABLE id, not the mutated lane", () => {
+    // Equal z + full overlap: order must be deterministic (id asc) and survive
+    // re-normalization — the historical oscillation bug tie-broke on the track.
+    const out = normalizeToZones([zClip("b", 0, 10, 0, 5), zClip("a", 0, 10, 0, 5)]);
+    expect(trackOf(out, "a")).toBe(0); // "a" < "b"
+    expect(trackOf(out, "b")).toBe(1);
+    const twice = normalizeToZones(out);
+    for (const e of out) expect(trackOf(twice, e.id)).toBe(e.track);
+  });
+
+  it("FIXED POINT: normalizeToZones(normalizeToZones(x)) === normalizeToZones(x) with z present", () => {
+    const input = [
+      zClip("hi", 0, 10, 0, 9),
+      zClip("lo", 0, 10, 0, 1),
+      zClip("mid", 2, 6, 0, 5),
+      zClip("seq", 12, 4, 0, 7),
+      zClip("music", 0, 16, 1, 3, "audio"),
+    ];
+    expectZoningIdempotent(input);
+  });
+
+  it("reload simulation: re-deriving lanes from the SAME z values yields identical lanes", () => {
+    // Simulate two independent discovery passes producing fresh element objects
+    // carrying the same z — lane assignment must be stable across reloads.
+    const build = (): TimelineElement[] => [
+      zClip("hi", 0, 10, 0, 9),
+      zClip("lo", 0, 10, 0, 1),
+      zClip("mid", 3, 5, 0, 5),
+    ];
+    const first = normalizeToZones(build());
+    const second = normalizeToZones(build());
+    for (const e of first) expect(trackOf(second, e.id)).toBe(e.track);
+  });
+});
+
+describe("normalizeToZones — cross-track z→lane (real qa-clean shape)", () => {
+  // Derived from /tmp/hf-dnd-qa/qa-clean: a full-length video on authored track 0
+  // (z=0), two logo SVGs on track 1 (z=26 and z=0), an icon on track 3 (z=5), and
+  // background music on track 2. In the canvas the z=26 / z=5 icons paint ON TOP of
+  // the z=0 video; the timeline must agree — the higher-z tracks sit on upper lanes.
+  const realProject = (): TimelineElement[] => [
+    zClip("ralu", 6.14, 3, 3, 5, "img"),
+    zClip("video", 1, 20, 0, 0, "video"),
+    zClip("blueLogo", 5.93, 3, 1, 26, "img"),
+    zClip("blackLogo", 1, 3, 1, 0, "img"),
+    zClip("music", 8.93, 8, 2, 0, "audio"),
+  ];
+
+  it("stacks a higher-z track ABOVE a lower-z track on a different authored track", () => {
+    const out = normalizeToZones(realProject());
+    // Track 1 (max z 26) tops the visual zone, then track 3 (z 5), then track 0 (z 0).
+    expect(trackOf(out, "blueLogo")).toBe(0);
+    expect(trackOf(out, "blackLogo")).toBe(0); // sequential to blueLogo → shares lane
+    expect(trackOf(out, "ralu")).toBe(1);
+    expect(trackOf(out, "video")).toBe(2);
+    // Audio stays at the very bottom regardless of its authored track index.
+    expect(trackOf(out, "music")).toBe(3);
+  });
+
+  it("the video (z=0) no longer sits above the z=26 / z=5 icons — canvas & timeline agree", () => {
+    const out = normalizeToZones(realProject());
+    expect(trackOf(out, "video")).toBeGreaterThan(trackOf(out, "blueLogo"));
+    expect(trackOf(out, "video")).toBeGreaterThan(trackOf(out, "ralu"));
+  });
+
+  it("is idempotent on the real-project shape (no lane drift on re-discovery)", () => {
+    const once = normalizeToZones(realProject());
+    const twice = normalizeToZones(once);
+    for (const e of once) expect(trackOf(twice, e.id)).toBe(e.track);
+  });
+
+  it("re-derives identical lanes from fresh objects carrying the same z (reload-stable)", () => {
+    const first = normalizeToZones(realProject());
+    const second = normalizeToZones(realProject());
+    for (const e of first) expect(trackOf(second, e.id)).toBe(e.track);
+  });
+
+  it("all-equal-z overlapping clips stack by DOM order (later on top), lanes contiguous", () => {
+    // Was: "keeps ascending authored track order when all tracks share z" — that
+    // pinned t0=0/t1=1/t3=2 to the authored track index. But these three all
+    // start at 0 and OVERLAP, all z=0, so CSS paints them by DOM order: t3 (last)
+    // on top. The per-clip pack now reflects that (t3 lane 0, then t1, then t0),
+    // which is what the canvas actually shows. Lanes stay contiguous 0..2.
+    const out = normalizeToZones([
+      zClip("t0", 0, 2, 0, 0),
+      zClip("t1", 0, 2, 1, 0),
+      zClip("t3", 0, 2, 3, 0),
+    ]);
+    expect(trackOf(out, "t3")).toBe(0); // last in DOM → paints on top
+    expect(trackOf(out, "t1")).toBe(1);
+    expect(trackOf(out, "t0")).toBe(2);
+  });
+});
+
+describe("normalizeToZones — EXACT qa-clean repro (per-clip constrained pack)", () => {
+  // The live repro from /tmp/hf-dnd-qa/qa-clean/index.html:
+  //   blue-logo  authored track 0, z=3, 6.37–9.37
+  //   ralu image authored track 0, z=0, 6.37–9.37   (shares track 0 with blue-logo)
+  //   black-logo authored track 1, z=1, 11.92–14.92
+  //   video      authored track 3, z=2, 0.84–20.84
+  // Canvas truth: video (z=2) covers ralu (z=0). The OLD whole-track packer
+  // ordered track 0 by its MAX z (3, from blue-logo), so ralu rode above the
+  // z=2 video — the timeline↔canvas contradiction. Array order below = DOM order.
+
+  it("ACCEPTANCE: lane order top→bottom is blue-logo, video, black-logo, ralu", () => {
+    const out = normalizeToZones(qaCleanRepro());
+    expect(trackOf(out, "blue-logo")).toBe(0); // z=3 → top
+    expect(trackOf(out, "video")).toBe(1); // z=2, overlaps blue-logo → below it
+    expect(trackOf(out, "black-logo")).toBe(2); // z=1, overlaps video (11.92–14.92 ∩ 0.84–20.84)
+    expect(trackOf(out, "ralu")).toBe(3); // z=0, overlaps blue-logo AND video → bottom
+  });
+
+  it("REGRESSION: a low-z clip must not ride its authored trackmate's high z above a clip that covers it", () => {
+    const out = normalizeToZones(qaCleanRepro());
+    // ralu (z=0) shares authored track 0 with blue-logo (z=3) but must sink BELOW
+    // the video (z=2) that overlaps and paints over it — the whole-track bug.
+    expect(trackOf(out, "ralu")).toBeGreaterThan(trackOf(out, "video"));
+    // black-logo (z=1) below video (z=2) because they overlap in time.
+    expect(trackOf(out, "black-logo")).toBeGreaterThan(trackOf(out, "video"));
+  });
+
+  it("FIXED POINT: running the NEW pack on its own output changes nothing", () => {
+    const once = normalizeToZones(qaCleanRepro());
+    const twice = normalizeToZones(once);
+    for (const e of once) expect(trackOf(twice, e.id)).toBe(e.track);
+    // And a third pass, to be sure convergence is genuine.
+    const thrice = normalizeToZones(twice);
+    for (const e of twice) expect(trackOf(thrice, e.id)).toBe(e.track);
+  });
+});
+
+describe("z ↔ lane round-trip convergence (both directions agree)", () => {
+  // Project a normalized TimelineElement onto the StackingElement view the
+  // forward (lane→z) mapping reasons over.
+  const toStacking = (els: TimelineElement[]): StackingElement[] =>
+    els.map((e, domIndex) => ({
+      key: e.key ?? e.id,
+      start: e.start,
+      duration: e.duration,
+      track: e.track,
+      zIndex: Number.isFinite(e.zIndex) ? (e.zIndex as number) : 0,
+      isAudio: classifyZone(e) === "audio",
+      domIndex,
+    }));
+
+  it("lane-move → z patch → re-discovery orders lanes by that same z → identical lanes (no oscillation)", () => {
+    // Two fully-overlapping visual clips. Authored: a below (z=1), b above (z=5).
+    const authored: TimelineElement[] = [zClip("a", 0, 10, 0, 1), zClip("b", 0, 10, 0, 5)];
+    const normalized = normalizeToZones(authored);
+    // z→lane placed b (z=5) on the upper lane 0, a on lane 1.
+    expect(trackOf(normalized, "b")).toBe(0);
+    expect(trackOf(normalized, "a")).toBe(1);
+
+    // USER lane-move: drag a to the TOP (lane 0) and push b down (lane 1).
+    const afterMove = normalized.map((e) =>
+      e.id === "a" ? { ...e, track: 0 } : e.id === "b" ? { ...e, track: 1 } : e,
+    );
+
+    // FORWARD: a lane-move writes the minimal z patch for the edited clip.
+    const patches = computeStackingPatches(toStacking(afterMove), ["a"]);
+    expect(patches).toEqual([{ key: "a", zIndex: 6 }]); // lifted above b (5)
+
+    // Apply the z patch back onto the elements (what handleDomZIndexReorderCommit
+    // persists; next discovery re-reads it as TimelineElement.zIndex).
+    const rediscovered = afterMove.map((e) => {
+      const p = patches.find((pp) => pp.key === (e.key ?? e.id));
+      return p ? { ...e, zIndex: p.zIndex } : e;
+    });
+
+    // REVERSE: re-normalize from the new z. a (z=6) must now own the upper lane —
+    // the same lane the user moved it to. Directions converge, they do not fight.
+    const renormalized = normalizeToZones(rediscovered);
+    expect(trackOf(renormalized, "a")).toBe(0);
+    expect(trackOf(renormalized, "b")).toBe(1);
+
+    // FIXED POINT: forward on the converged state produces NO further patch, and
+    // reverse is idempotent — the round-trip is stable.
+    expect(computeStackingPatches(toStacking(renormalized), ["a"])).toEqual([]);
+    const twice = normalizeToZones(renormalized);
+    for (const e of renormalized) expect(trackOf(twice, e.id)).toBe(e.track);
+  });
+
+  it("qa-clean: drag video BELOW ralu → z patch → re-pack keeps it below, no oscillation", () => {
+    // EXACT repro fixture (array order = DOM order).
+    const normalized = normalizeToZones(qaCleanRepro());
+    // Baseline lanes: blue-logo 0, video 1, black-logo 2, ralu 3.
+    expect(trackOf(normalized, "video")).toBe(1);
+    expect(trackOf(normalized, "ralu")).toBe(3);
+
+    // USER lane-move: drag video to the lane BELOW ralu (bottom). ralu is at
+    // lane 3, so video goes to a lane strictly greater — model it as lane 4.
+    const afterMove = normalized.map((e) => (e.id === "video" ? { ...e, track: 4 } : e));
+
+    // FORWARD: video (z=2) must drop below ralu (z=0). No integer z ≥ 0 fits
+    // strictly below 0, so the tie-aware sync cascades: video→0 and the clips that
+    // must stay above it (ralu z=0, black-logo z=1, blue-logo z=3) are bumped as
+    // needed so video paints below ralu with all z ≥ 0.
+    const patches = computeStackingPatches(toStacking(afterMove), ["video"]);
+    const patchByKey = new Map(patches.map((p) => [p.key, p.zIndex]));
+    // Video was moved; it must now be strictly below ralu in paint order.
+    const zAfter = (id: string): number =>
+      patchByKey.get(id) ?? (afterMove.find((e) => e.id === id)!.zIndex as number);
+    expect(zAfter("video")).toBeLessThan(zAfter("ralu"));
+    expect(zAfter("video")).toBeGreaterThanOrEqual(0);
+    expect(patchByKey.size).toBeGreaterThan(0);
+
+    // Apply patches and re-pack: video's lane must now be BELOW ralu's.
+    const rediscovered = afterMove.map((e) => {
+      const z = patchByKey.get(e.id);
+      return z != null ? { ...e, zIndex: z } : e;
+    });
+    const renormalized = normalizeToZones(rediscovered);
+    expect(trackOf(renormalized, "video")).toBeGreaterThan(trackOf(renormalized, "ralu"));
+
+    // FIXED POINT: re-running BOTH directions on the converged state is a no-op.
+    expect(computeStackingPatches(toStacking(renormalized), ["video"])).toEqual([]);
+    const twice = normalizeToZones(renormalized);
+    for (const e of renormalized) expect(trackOf(twice, e.id)).toBe(e.track);
+  });
+});

packages/studio/src/player/components/timelineZones.ts

@@ -0,0 +1,198 @@
+import type { TimelineElement } from "../store/playerStore";
+import { isAudioTimelineElement } from "../../utils/timelineInspector";
+
+/**
+ * Free-form vertical zones, top → bottom: visual, audio. There is no "main track"
+ * — layering is CSS z-index (the renderer ignores track index), so the timeline's
+ * only job is to keep visual clips grouped above audio clips.
+ */
+export type TrackZone = "visual" | "audio";
+
+/** Which zone a clip belongs to: audio elements sink to the bottom, everything
+ *  else (video / image / text / sub-comp) is a visual lane on top. */
+export function classifyZone(el: TimelineElement): TrackZone {
+  return isAudioTimelineElement(el) ? "audio" : "visual";
+}
+
+const keyOf = (el: TimelineElement) => el.key ?? el.id;
+
+/** Stacking order for a clip: missing / "auto" z is treated as 0. */
+const zOf = (el: TimelineElement) => (Number.isFinite(el.zIndex) ? (el.zIndex as number) : 0);
+
+const EPS = 1e-6;
+
+/** Two clips overlap when their half-open [start, end) intervals intersect. */
+function overlaps(a: TimelineElement, b: TimelineElement): boolean {
+  return a.start < b.start + b.duration - EPS && b.start < a.start + a.duration - EPS;
+}
+
+/** A clip paired with its position in the discovery/document (input) order. */
+interface IndexedClip {
+  el: TimelineElement;
+  /** Index in the input `elements` array = discovery/DOM order. */
+  domIndex: number;
+}
+
+/** One display lane: the clips packed onto it, in placement order. */
+interface Lane {
+  occupants: IndexedClip[];
+  /** The single authored track all occupants share, or null once mixed (never
+   *  happens — we only ever add same-track clips to an existing lane). */
+  track: number;
+}
+
+/**
+ * Lowest lane index a clip may occupy: strictly above every already-placed lane
+ * holding a clip it overlaps in time (all of which out-stack it by the z-desc
+ * placement order).
+ */
+function lowestAllowedLane(lanes: Lane[], item: IndexedClip): number {
+  let minLane = 0;
+  for (let i = 0; i < lanes.length; i++) {
+    if (lanes[i].occupants.some((o) => overlaps(o.el, item.el))) minLane = i + 1;
+  }
+  return minLane;
+}
+
+/**
+ * First lane at index ≥ minLane that holds solely this clip's authored track and
+ * nothing overlapping (so sequential same-track clips share a lane); -1 when none
+ * qualifies and a fresh lane must open.
+ */
+function findReusableLane(lanes: Lane[], minLane: number, item: IndexedClip): number {
+  for (let i = minLane; i < lanes.length; i++) {
+    const lane = lanes[i];
+    if (lane.track !== item.el.track) continue;
+    if (lane.occupants.some((o) => overlaps(o.el, item.el))) continue;
+    return i;
+  }
+  return -1;
+}
+
+/**
+ * Pack a WHOLE zone's clips onto display lanes with a single constrained pass so
+ * that, for EVERY pair of time-overlapping clips, lane order (upper = lower index)
+ * equals canvas stacking order. This replaces the old two-stage
+ * `orderTrackBlocksByZ` + per-track `packTrackLanes`, which ordered whole authored
+ * tracks by their MAX z and so lifted a low-z clip above a clip that covers it
+ * whenever it shared a track with a high-z clip (the qa-clean ralu/video bug — a
+ * low-z image rode its z=3 trackmate above the z=2 video that paints over it). No
+ * whole-track mapping can fix that; the mapping must be per-clip.
+ *
+ * Algorithm:
+ *   1. Order clips by z DESC; z-tie → INPUT-ARRAY-INDEX (DOM order) DESC (CSS
+ *      paints equal-z siblings by DOM order — LATER in DOM paints on top, so it
+ *      must place first / upper); final tie → stable key. NEVER tie-break on the
+ *      mutated lane/track index (historic oscillation bug — the tie-break must be
+ *      a stable function of the input, not of the output being computed).
+ *   2. Place each clip at lane ≥ (1 + highest lane among already-placed clips it
+ *      OVERLAPS IN TIME). By z-desc placement every already-placed overlapping
+ *      clip out-stacks this one (higher z, or equal z but later in DOM), so this
+ *      guarantees lane order == stacking order for every overlapping pair.
+ *   3. To preserve the "distinct authored tracks stay distinct / sequential
+ *      same-track clips share a lane" feel, reuse an existing lane at index ≥ that
+ *      minimum ONLY when the lane's occupants are all from the SAME authored track
+ *      AND none overlaps this clip in time; otherwise open a fresh lane.
+ *
+ * Writes each clip's absolute display lane (`base + laneIndex`) into `laneOf` and
+ * returns the number of lanes used (≥ 1 when non-empty).
+ */
+function packZoneLanes(clips: IndexedClip[], base: number, laneOf: Map<string, number>): number {
+  const ordered = [...clips].sort(
+    (a, b) =>
+      zOf(b.el) - zOf(a.el) || b.domIndex - a.domIndex || (keyOf(a.el) < keyOf(b.el) ? -1 : 1),
+  );
+  const lanes: Lane[] = [];
+  for (const item of ordered) {
+    const minLane = lowestAllowedLane(lanes, item);
+    let placed = findReusableLane(lanes, minLane, item);
+    if (placed === -1) {
+      placed = lanes.length;
+      lanes.push({ occupants: [], track: item.el.track });
+    }
+    lanes[placed].occupants.push(item);
+    laneOf.set(keyOf(item.el), base + placed);
+  }
+  return lanes.length;
+}
+
+/**
+ * Legacy per-track interval packing for the AUDIO zone (no z semantics): pack one
+ * authored track's clips onto sub-lanes so no two overlap in time — sequential
+ * clips share a lane, overlapping ones spill onto the next (first-fit). Ordered by
+ * start (then stable key) so the layout is deterministic and idempotent. Returns
+ * the number of lanes used (≥ 1 when non-empty).
+ */
+function packAudioTrackLanes(
+  clips: IndexedClip[],
+  base: number,
+  laneOf: Map<string, number>,
+): number {
+  const ordered = [...clips].sort(
+    (a, b) => a.el.start - b.el.start || (keyOf(a.el) < keyOf(b.el) ? -1 : 1),
+  );
+  const lanes: IndexedClip[][] = [];
+  for (const item of ordered) {
+    let sub = lanes.findIndex((occ) => occ.every((o) => !overlaps(o.el, item.el)));
+    if (sub === -1) {
+      sub = lanes.length;
+      lanes.push([]);
+    }
+    lanes[sub].push(item);
+    laneOf.set(keyOf(item.el), base + sub);
+  }
+  return Math.max(1, lanes.length);
+}
+
+/**
+ * Assign display lanes for the timeline: visual lanes on top, audio lanes below.
+ *
+ * The VISUAL zone is packed per-clip (packZoneLanes) so the timeline's vertical
+ * order matches the canvas's CSS stacking for EVERY time-overlapping pair — a
+ * low-z clip sinks below a clip that covers it even if it shares an authored track
+ * with a higher-z clip. Time-overlapping clips still split onto separate lanes
+ * (standard NLE), sequential same-track clips still share a lane, and distinct
+ * authored tracks stay distinct.
+ *
+ * The AUDIO zone keeps the original behavior — authored-track order, per-track
+ * interval packing — because audio has no z / stacking semantics.
+ *
+ * Pure — returns a new array; unchanged clips keep their identity. Display-only
+ * (runs on discovery); it does not rewrite the source. Idempotent (running it on
+ * its own output is a fixed point).
+ */
+export function normalizeToZones(elements: TimelineElement[]): TimelineElement[] {
+  if (elements.length === 0) return elements;
+
+  const laneOf = new Map<string, number>();
+  let nextLane = 0;
+
+  const visual: IndexedClip[] = [];
+  const audio: IndexedClip[] = [];
+  elements.forEach((el, domIndex) => {
+    (classifyZone(el) === "audio" ? audio : visual).push({ el, domIndex });
+  });
+
+  nextLane += packZoneLanes(visual, nextLane, laneOf);
+
+  // Audio: preserve legacy behavior — group by authored track (ascending), pack
+  // each track's overlapping clips onto sub-lanes.
+  const audioByTrack = new Map<number, IndexedClip[]>();
+  for (const item of audio) {
+    const list = audioByTrack.get(item.el.track);
+    if (list) list.push(item);
+    else audioByTrack.set(item.el.track, [item]);
+  }
+  for (const track of [...audioByTrack.keys()].sort((a, b) => a - b)) {
+    nextLane += packAudioTrackLanes(audioByTrack.get(track)!, nextLane, laneOf);
+  }
+
+  let changed = false;
+  const remapped = elements.map((el) => {
+    const lane = laneOf.get(keyOf(el));
+    if (lane == null || lane === el.track) return el;
+    changed = true;
+    return { ...el, track: lane };
+  });
+  return changed ? remapped : elements;
+}