feat(studio): timeline zones + asset-click/nudge policy (pr08+pr09 recovery)

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

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+import type { TimelineElement } from "../store/playerStore";
+
+/**
+ * Keep a landing track inside the dragged clip's kind-zone: visual clips stay in
+ * the rows ABOVE the first audio lane; audio clips stay AT/BELOW it. Prevents a
+ * clip from appearing to land in the wrong zone mid-drag (which normalizeToZones
+ * would then snap back). `audioRow` = index in `trackOrder` of the first audio
+ * lane, or -1 when there is no audio zone yet (then it's a no-op).
+ */
+export function clampTrackToZone(
+  targetTrack: number,
+  trackOrder: number[],
+  audioRow: number,
+  isAudio: boolean,
+): number {
+  if (audioRow < 0) return targetTrack;
+  const row = trackOrder.indexOf(targetTrack);
+  if (row < 0) return targetTrack;
+  if (isAudio) return row >= audioRow ? targetTrack : (trackOrder[audioRow] ?? targetTrack);
+  return row < audioRow ? targetTrack : (trackOrder[audioRow - 1] ?? targetTrack);
+}
+
+/**
+ * Whether a new-track insert at boundary `insertRow` is allowed for a clip of the
+ * given kind. Visual clips may only insert visual lanes (boundary at/above the top
+ * of the audio zone); audio clips may only insert audio lanes (boundary at/below
+ * it) — so audio clips CAN create a new audio track, and neither kind inserts into
+ * the other's zone. `audioRow` = first audio lane row, or -1 (no audio zone) → any.
+ */
+export function isInsertAllowedForZone(
+  insertRow: number,
+  audioRow: number,
+  isAudio: boolean,
+): boolean {
+  if (audioRow < 0) return true;
+  return isAudio ? insertRow >= audioRow : insertRow <= audioRow;
+}
+
+/**
+ * The full drop-placement decision for a dragged clip — one pure, testable unit.
+ * Enforces: NO time-overlap on a single track; a clip stays in its kind-zone;
+ * a new track is created only when needed. Order of resolution:
+ *   1. Deliberate boundary insert (pointer near a lane edge), if it's in the
+ *      clip's own zone → create a new track there.
+ *   2. Otherwise land on a lane: clamp the aimed track to the clip's zone, take it
+ *      if free at [start, start+duration), else the nearest FREE lane in the zone
+ *      (prefer up), else auto-create a new track right below the aimed lane.
+ * `audioTracks` = the set of track indices that currently hold audio (so the fn
+ * needs no element-kind import). Returns the landing `track` and, when a new track
+ * should be created, the `insertRow` boundary (else null).
+ *
+ * `preferInsertAbove` biases the auto-created track (occupied-aim → new adjacent
+ * track) toward the boundary ABOVE the aimed row instead of below it, so the new
+ * lane opens on whichever side of the aimed clip the pointer is nearer (the drag
+ * preview passes the pointer's sub-row half). A clip whose aimed span is occupied
+ * never snaps back to its origin — it relocates to a free lane, or (none free)
+ * gets a fresh track next to the aim. Default (below) preserves prior behaviour.
+ */
+/**
+ * Insert-row boundary for an out-of-range aim — a `desired` track that isn't a
+ * real lane: the sentinel minTrack-1 an upward create-drag emits (#2214-adjacent
+ * repro) or a beyond-the-bottom index a downward one does. Anchors the new track
+ * to a boundary of the clip's OWN kind-zone so a visual insert can never land
+ * past the audio zone (the old below = order.length fallback dropped it BELOW the
+ * audio lanes). Above the zone (`desired` < the zone's min lane) → the zone's TOP
+ * boundary; otherwise → its BOTTOM boundary (for a visual clip, the top of the
+ * audio zone). `zoneTracks` = this kind's lanes, in `order` sequence.
+ */
+function outOfRangeZoneInsertRow(
+  order: number[],
+  zoneTracks: number[],
+  audioRow: number,
+  desired: number,
+): number {
+  // No lane of this kind yet: fall to the split (audioRow) or the very top.
+  // A visual-only timeline has audioRow -1 (top); an all-audio one has it at 0.
+  if (zoneTracks.length === 0) return audioRow < 0 ? 0 : audioRow;
+  // zoneTracks preserves `order` sequence, so its ends map to the zone boundary
+  // rows: above the zone's min lane → its top boundary, else its bottom.
+  const zoneTop = order.indexOf(zoneTracks[0]);
+  const zoneBottom = order.indexOf(zoneTracks[zoneTracks.length - 1]) + 1;
+  return desired < Math.min(...zoneTracks) ? zoneTop : zoneBottom;
+}
+
+export function resolveZoneDropPlacement(input: {
+  order: number[];
+  audioTracks: ReadonlySet<number>;
+  elements: TimelineElement[];
+  desiredTrack: number;
+  deliberateInsertRow: number | null;
+  start: number;
+  duration: number;
+  dragKey: string;
+  isAudio: boolean;
+  preferInsertAbove?: boolean;
+}): { track: number; insertRow: number | null } {
+  const { order, audioTracks, elements, desiredTrack, deliberateInsertRow } = input;
+  const { start, duration, dragKey, isAudio, preferInsertAbove } = input;
+  const audioRow = order.findIndex((t) => audioTracks.has(t));
+
+  if (
+    deliberateInsertRow !== null &&
+    isInsertAllowedForZone(deliberateInsertRow, audioRow, isAudio)
+  ) {
+    return { track: desiredTrack, insertRow: deliberateInsertRow };
+  }
+
+  const desired = clampTrackToZone(desiredTrack, order, audioRow, isAudio);
+  const zoneTracks = order.filter((t) => audioTracks.has(t) === isAudio);
+  const placement = resolvePlacement({
+    elements,
+    desiredTrack: desired,
+    start,
+    duration,
+    trackOrder: zoneTracks,
+    excludeKey: dragKey,
+  });
+  if (placement.needsInsert) {
+    const desiredRow = order.indexOf(desired);
+    if (desiredRow < 0) {
+      return {
+        track: desired,
+        insertRow: outOfRangeZoneInsertRow(order, zoneTracks, audioRow, desired),
+      };
+    }
+    // Prefer the gap NEAREST the pointer: insert above the aimed row when the
+    // pointer sits in its upper half AND that boundary is in the clip's own zone
+    // (else the visual/audio split would be crossed) — otherwise fall to below.
+    // `desired` is clamped into the zone, so both boundaries stay in-zone.
+    const insertRow =
+      preferInsertAbove && isInsertAllowedForZone(desiredRow, audioRow, isAudio)
+        ? desiredRow
+        : desiredRow + 1;
+    return { track: desired, insertRow };
+  }
+  return { track: placement.track, insertRow: null };
+}
+
+/**
+ * Fallback half-width (fraction of a track height) of the insert band straddling
+ * a lane boundary — used only when the caller passes no explicit band. Production
+ * threads the geometry-exact `INSERT_BOUNDARY_BAND` (timelineLayout.ts, = the clip
+ * inset `CLIP_Y / TRACK_H`) so the band matches the rendered inter-clip gutter and
+ * NEVER reaches into a clip body. Kept in sync with that constant; do not widen it
+ * back toward the old 0.32 (which armed an insert across ~64% of every row — the
+ * misfire that turned a plain horizontal drag into a phantom track insert).
+ */
+const INSERT_BAND = 3 / 48;
+
+/**
+ * Decide whether a vertical drag is inserting a new track at a lane boundary.
+ * `rowFloat` is the pointer's position in track-height units from the top of the
+ * first lane (0 = top of lane 0). Returns the boundary row to insert at
+ * (0 = above the top lane, `trackCount` = below the bottom), or null when the
+ * pointer is over a lane's middle band (a normal move/target).
+ */
+export function resolveInsertRow(
+  rowFloat: number,
+  trackCount: number,
+  band: number = INSERT_BAND,
+): number | null {
+  if (trackCount === 0) return 0;
+  if (rowFloat <= 0) return 0;
+  if (rowFloat >= trackCount) return trackCount;
+  const lane = Math.floor(rowFloat);
+  const frac = rowFloat - lane;
+  if (frac < band) return lane;
+  if (frac > 1 - band) return lane + 1;
+  return null;
+}
+
+/** Half-open overlap test: [aStart, aEnd) intersects [bStart, bEnd). */
+export function timeRangesOverlap(
+  aStart: number,
+  aEnd: number,
+  bStart: number,
+  bEnd: number,
+): boolean {
+  return aStart < bEnd && bStart < aEnd;
+}
+
+/**
+ * True when no clip on `track` overlaps [start, end) — excluding the clip
+ * identified by `excludeKey` (the one being dragged).
+ */
+export function isLaneFree(
+  elements: TimelineElement[],
+  track: number,
+  start: number,
+  end: number,
+  excludeKey: string | null,
+): boolean {
+  return !elements.some(
+    (el) =>
+      (el.key ?? el.id) !== excludeKey &&
+      el.track === track &&
+      timeRangesOverlap(start, end, el.start, el.start + el.duration),
+  );
+}
+
+export interface PlacementInput {
+  elements: TimelineElement[];
+  desiredTrack: number;
+  start: number;
+  duration: number;
+  trackOrder: number[];
+  excludeKey: string | null;
+}
+
+export interface PlacementResult {
+  /** The lane the clip should land on. */
+  track: number;
+  /**
+   * True when no existing lane was free and the caller should insert a new
+   * track instead of landing on `track` (which is then the desired lane as a
+   * last-resort fallback). Consumed in later stages (2b/2c); stage 2a ignores it.
+   */
+  needsInsert: boolean;
+}
+
+/**
+ * Resolve where a dragged clip should land, avoiding overlap. If the desired
+ * lane is free, keep it. Otherwise search the nearest free lane, **preferring
+ * up** (all lanes above, nearest first), then down. If none is free, signal an
+ * insert and fall back to the desired lane.
+ */
+export function resolvePlacement({
+  elements,
+  desiredTrack,
+  start,
+  duration,
+  trackOrder,
+  excludeKey,
+}: PlacementInput): PlacementResult {
+  const end = start + duration;
+  const idx = trackOrder.indexOf(desiredTrack);
+  // desiredTrack is not one of the zone's lanes — the clip's kind-zone has no lane
+  // yet (e.g. an audio clip dropped on a visual-only timeline). This MUST be checked
+  // BEFORE the isLaneFree short-circuit below: a free-aimed span on a foreign-zone
+  // lane (an audio clip aimed at an empty stretch of a visual-only timeline) is
+  // "free" only because that lane belongs to the wrong zone. Landing there would
+  // put the clip in the wrong kind-zone, so signal an insert to create the zone's
+  // first lane instead — regardless of whether the aimed span is occupied (#2195).
+  if (idx === -1) return { track: desiredTrack, needsInsert: true };
+
+  if (isLaneFree(elements, desiredTrack, start, end, excludeKey)) {
+    return { track: desiredTrack, needsInsert: false };
+  }
+
+  // Prefer up: nearest lane above first, then the rest above.
+  for (let up = idx - 1; up >= 0; up--) {
+    if (isLaneFree(elements, trackOrder[up], start, end, excludeKey)) {
+      return { track: trackOrder[up], needsInsert: false };
+    }
+  }
+  // Then down: nearest lane below first.
+  for (let down = idx + 1; down < trackOrder.length; down++) {
+    if (isLaneFree(elements, trackOrder[down], start, end, excludeKey)) {
+      return { track: trackOrder[down], needsInsert: false };
+    }
+  }
+  return { track: desiredTrack, needsInsert: true };
+}

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

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+import { describe, it, expect } from "vitest";
+import {
+  clampGroupMoveDelta,
+  isMultiDragActive,
+  isMultiDragPassenger,
+  multiDragDeltaSeconds,
+  multiDragPassengerOffsetPx,
+  type MultiDragPreviewInput,
+} from "./timelineMultiDragPreview";
+
+const base = (over: Partial<MultiDragPreviewInput> = {}): MultiDragPreviewInput => ({
+  dragStarted: true,
+  draggedKey: "a",
+  draggedOriginStart: 2,
+  draggedPreviewStart: 5,
+  selectedKeys: new Set(["a", "b", "c"]),
+  ...over,
+});
+
+describe("isMultiDragActive", () => {
+  it("is active when a started drag's clip is part of a 2+ selection", () => {
+    expect(isMultiDragActive(base())).toBe(true);
+  });
+
+  it("is inactive before the drag starts", () => {
+    expect(isMultiDragActive(base({ dragStarted: false }))).toBe(false);
+  });
+
+  it("is inactive for a single-clip selection (single-drag behavior)", () => {
+    expect(isMultiDragActive(base({ selectedKeys: new Set(["a"]) }))).toBe(false);
+  });
+
+  it("is inactive when the dragged clip is not itself selected", () => {
+    expect(isMultiDragActive(base({ draggedKey: "z" }))).toBe(false);
+  });
+});
+
+describe("multiDragDeltaSeconds (the one formation delta)", () => {
+  it("is the grabbed clip's preview − origin start when active", () => {
+    // The preview start is already group-clamped upstream, so this delta is the
+    // clamped delta every member (ghost + passengers) moves by.
+    expect(multiDragDeltaSeconds(base())).toBe(3);
+  });
+
+  it("supports a leftward (negative) delta", () => {
+    expect(multiDragDeltaSeconds(base({ draggedPreviewStart: 0.5 }))).toBeCloseTo(-1.5);
+  });
+
+  it("is zero when no multi-drag is active", () => {
+    expect(multiDragDeltaSeconds(base({ selectedKeys: new Set(["a"]) }))).toBe(0);
+  });
+});
+
+describe("isMultiDragPassenger", () => {
+  it("marks a selected non-dragged clip as a passenger", () => {
+    expect(isMultiDragPassenger("b", base())).toBe(true);
+    expect(isMultiDragPassenger("c", base())).toBe(true);
+  });
+
+  it("never marks the dragged clip itself (it is the free ghost)", () => {
+    expect(isMultiDragPassenger("a", base())).toBe(false);
+  });
+
+  it("never marks an unselected clip", () => {
+    expect(isMultiDragPassenger("d", base())).toBe(false);
+  });
+
+  it("marks nothing when the drag is a single-drag", () => {
+    const single = base({ selectedKeys: new Set(["a"]) });
+    expect(isMultiDragPassenger("b", single)).toBe(false);
+  });
+});
+
+describe("multiDragPassengerOffsetPx (rigid: every passenger shares the delta)", () => {
+  it("converts the one formation delta to pixels for every passenger", () => {
+    // Both passengers move by the SAME 3s × 100pps = 300px — spacing locked.
+    expect(multiDragPassengerOffsetPx("b", 100, base())).toBe(300);
+    expect(multiDragPassengerOffsetPx("c", 100, base())).toBe(300);
+  });
+
+  it("is zero for the dragged clip and for non-passengers", () => {
+    expect(multiDragPassengerOffsetPx("a", 100, base())).toBe(0);
+    expect(multiDragPassengerOffsetPx("d", 100, base())).toBe(0);
+  });
+
+  it("is zero for a non-finite pps", () => {
+    expect(multiDragPassengerOffsetPx("b", Number.NaN, base())).toBe(0);
+  });
+
+  it("follows a leftward delta", () => {
+    expect(multiDragPassengerOffsetPx("c", 50, base({ draggedPreviewStart: 0 }))).toBe(-100);
+  });
+});
+
+describe("clampGroupMoveDelta (rigid group move)", () => {
+  it("passes a rightward delta through unchanged (no right wall)", () => {
+    expect(clampGroupMoveDelta(3, [2, 5, 9])).toBe(3);
+    expect(clampGroupMoveDelta(1000, [0, 4])).toBe(1000);
+  });
+
+  it("passes a leftward delta through when no member would cross 0", () => {
+    // Leftmost member at 5, moving left by 3 → 2 ≥ 0, so unclamped.
+    expect(clampGroupMoveDelta(-3, [5, 8, 12])).toBe(-3);
+  });
+
+  it("clamps a leftward delta so the leftmost member stops exactly at 0", () => {
+    // Leftmost at 2 → the furthest left the group can move is -2 (that member → 0).
+    // A pointer asking for -5 is clamped to -2: the grabbed clip stops with the
+    // formation instead of out-running it.
+    expect(clampGroupMoveDelta(-5, [2, 6, 10])).toBe(-2);
+  });
+
+  it("is bounded by the MOST-constrained (leftmost) member, not the grabbed one", () => {
+    // Grabbed clip is at 10; a passenger at 1 is the constraint. Max left = -1.
+    expect(clampGroupMoveDelta(-8, [10, 1, 4])).toBe(-1);
+  });
+
+  it("already-at-0 member forbids any leftward move", () => {
+    expect(clampGroupMoveDelta(-4, [0, 3, 7])).toBe(0);
+    // rightward still allowed
+    expect(clampGroupMoveDelta(2, [0, 3, 7])).toBe(2);
+  });
+
+  it("returns the raw delta for an empty formation", () => {
+    expect(clampGroupMoveDelta(-9, [])).toBe(-9);
+  });
+});