fix(studio): reconcile keyframe-seek timing basis between flat Layout and Motion groups

Whole-plan coherence review (Plan 3a Layout + Plan 3b Motion) found that
Layout's keyframe gutter and Motion's Timing row independently derived an
element's start/duration and disagreed whenever an element had animations
but no explicit data-duration: Motion correctly inferred the range from the
element's GSAP tweens, while Layout's keyframe gutter fell back to a naive
`duration ?? 1`, so clicking a keyframe percentage in Layout could seek to a
different absolute time than what Motion's Timing row displayed.

Extract deriveElementTiming (propertyPanelFlatTimingDerivation.ts) as the
single shared basis both paths now consume: FlatTimingRow (Motion) and
PropertyPanelFlat's own elStart/elDuration (Layout's keyframe gutter and 3D
Transform block). PropertyPanelFlat now recomputes this basis itself from
its own element/gsapAnimations props instead of trusting the parent's naive
value, so PropertyPanel.tsx (and its legacy non-flat panel) is untouched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/studio/src/components/editor/PropertyPanel.test.tsx

@@ -143,6 +143,40 @@ function animatedElement() {
   };
 }
 
+// Inferred-timing fixture (whole-plan coherence fix): NO explicit data-start
+// or data-duration — sections.timing must turn on via animationCount (fed
+// from gsapAnimations.length), not an authored attribute, so both the Motion
+// Timing row and the Layout keyframe gutter are forced to infer the range
+// from the element's own GSAP tween instead of reading it off an attribute.
+function inferredMotionElement() {
+  return {
+    ...baseElement(),
+    id: "inferred-anim",
+    selector: "#inferred-anim",
+    label: "Inferred Anim",
+  };
+}
+
+// A single "to" tween running from t=2 to t=5 (position 2, duration 3), with
+// keyframes on "x" at 0/50/100% — enough to drive both FlatTimingRow's
+// inference and the Layout "x" row's keyframe-seek gutter.
+const INFERRED_TIMING_ANIMATION = {
+  id: "a1",
+  targetSelector: "#inferred-anim",
+  method: "to",
+  position: 2,
+  duration: 3,
+  properties: { x: 100 },
+  keyframes: {
+    format: "percentage",
+    keyframes: [
+      { percentage: 0, properties: { x: 0 } },
+      { percentage: 50, properties: { x: 50 } },
+      { percentage: 100, properties: { x: 100 } },
+    ],
+  },
+} as never;
+
 async function renderPanel(
   flatEnabled: boolean,
   elementOverride: ReturnType<typeof baseElement> = baseElement(),
@@ -418,3 +452,65 @@ describe("PropertyPanel — Motion group (Plan 3b)", () => {
     RENDER_TIMEOUT_MS,
   );
 });
+
+// Whole-plan coherence fix: Layout's keyframe-seek basis and Motion's Timing
+// row basis must agree on the same start/duration for an element that has
+// animations but no explicit data-duration — before the fix, Layout fell back
+// to a naive `duration ?? 1` while Motion correctly inferred the range from
+// the tween (position 2, duration 3 -> start 2 / duration 3 / end 5).
+describe("PropertyPanel — flat Layout/Motion timing agreement (whole-plan coherence fix)", () => {
+  it(
+    "Motion's Timing row shows the inferred start/end/duration for an element with animations but no explicit duration",
+    async () => {
+      const { host, root } = await renderPanel(true, inferredMotionElement(), {
+        gsapAnimations: [INFERRED_TIMING_ANIMATION],
+      });
+      openFlatGroup(host, "Motion");
+      const motionGroup = host.querySelector('[data-flat-group-open="true"]');
+      if (!motionGroup) throw new Error("expected the Motion group to be open");
+      expect(motionGroup.textContent).toContain("Inferred");
+      const inputs = motionGroup.querySelectorAll<HTMLInputElement>("input");
+      // FlatTimingRow renders Start, End, Duration in that order.
+      expect(inputs[0]?.value).toBe("2.00s");
+      expect(inputs[1]?.value).toBe("5.00s");
+      expect(inputs[2]?.value).toBe("3.00s");
+      act(() => root.unmount());
+    },
+    RENDER_TIMEOUT_MS,
+  );
+
+  it(
+    "Layout's X-row keyframe gutter seeks to the SAME absolute time Motion's Timing row shows as the midpoint (50% of an inferred 2s-5s range = 3.5s)",
+    async () => {
+      const onSeekToTime = vi.fn();
+      const { host, root } = await renderPanel(true, inferredMotionElement(), {
+        gsapAnimations: [INFERRED_TIMING_ANIMATION],
+        onSeekToTime,
+      });
+      openFlatGroup(host, "Layout");
+      const layoutGroup = host.querySelector('[data-flat-group-open="true"]');
+      if (!layoutGroup) throw new Error("expected the Layout group to be open");
+
+      const xRow = Array.from(layoutGroup.querySelectorAll<HTMLElement>(".group")).find(
+        (el) => el.querySelector("span")?.textContent === "X",
+      );
+      if (!xRow) throw new Error("expected an X row");
+      const gutter = xRow.querySelector('[data-flat-kf-gutter="true"]');
+      if (!gutter) throw new Error("expected a keyframe gutter on the X row");
+      // The diamond button always carries a `title`; the two plain arrow
+      // buttons don't. At currentPct=0 with keyframes at 0/50/100%, the prev
+      // arrow is disabled (no earlier keyframe) and the next arrow seeks to
+      // the 50% keyframe — exactly the case the coherence bug affected.
+      const nextArrow = Array.from(gutter.querySelectorAll<HTMLButtonElement>("button")).find(
+        (b) => !b.title && !b.disabled,
+      );
+      if (!nextArrow) throw new Error("expected an enabled next-keyframe arrow button");
+      act(() => nextArrow.dispatchEvent(new MouseEvent("click", { bubbles: true })));
+
+      // Same basis as the Timing row: start 2 + 50% * duration 3 = 3.5.
+      expect(onSeekToTime).toHaveBeenCalledWith(3.5);
+      act(() => root.unmount());
+    },
+    RENDER_TIMEOUT_MS,
+  );
+});

packages/studio/src/components/editor/PropertyPanelFlat.tsx

@@ -11,6 +11,7 @@ import { FlatTextSection } from "./propertyPanelFlatTextSection";
 import { FlatStyleSection } from "./propertyPanelFlatStyleSections";
 import { FlatLayoutSection } from "./propertyPanelFlatLayoutSection";
 import { FlatMotionSection } from "./propertyPanelFlatMotionSection";
+import { deriveElementTiming } from "./propertyPanelFlatTimingDerivation";
 import { createGsapLivePreview } from "./gsapLivePreview";
 import { formatTextFieldPreview, StyleSections } from "./propertyPanelSections";
 import { STUDIO_GSAP_PANEL_ENABLED } from "./manualEditingAvailability";
@@ -96,8 +97,12 @@ export function PropertyPanelFlat({
   currentPct,
   animIdForProp,
   gsapRuntimeValues,
-  elStart,
-  elDuration,
+  // Renamed: PropertyPanel.tsx still computes/passes these for its own legacy
+  // (non-flat) panel, but the flat path recomputes its own basis below via
+  // deriveElementTiming so it agrees with Motion's Timing row — ignore the
+  // parent's naive `elDuration ?? 1` fallback.
+  elStart: _elStart,
+  elDuration: _elDuration,
   onCommitAnimatedProperty,
   onCommitAnimatedProperties,
   onSeekToTime,
@@ -222,6 +227,12 @@ export function PropertyPanelFlat({
     setPinnedGroupIds((current) =>
       current.includes(groupId) ? current.filter((id) => id !== groupId) : [...current, groupId],
     );
+  // Basis for the Layout keyframe gutter (X/Y/W/H/Angle + 3D Transform) —
+  // must agree with Motion's Timing row (FlatTimingRow), which infers the
+  // range from animations when there's no explicit data-duration. Computed
+  // here (not threaded from PropertyPanel) both to keep that file under its
+  // 600-LOC gate and because element/gsapAnimations are already in scope.
+  const { start: elStart, duration: elDuration } = deriveElementTiming(element, gsapAnimations);
   // Trivial percentage→time seek, derived here rather than threaded from
   // PropertyPanel (keeps that file under its 600-LOC gate).
   const seekFromKfPct = (pct: number) => onSeekToTime?.(elStart + (pct / 100) * elDuration);

packages/studio/src/components/editor/propertyPanelFlatMotionSection.tsx

@@ -7,21 +7,7 @@ import { CommitField } from "./propertyPanelPrimitives";
 import { AnimationCard } from "./AnimationCard";
 import { ADD_METHODS, ADD_METHOD_LABELS, METHOD_TOOLTIPS } from "./gsapAnimationConstants";
 import type { GsapAnimationEditCallbacks } from "./gsapAnimationCallbacks";
-
-function deriveTimingFromAnimations(
-  animations: GsapAnimation[],
-): { start: number; duration: number } | null {
-  let lo = Infinity;
-  let hi = -Infinity;
-  for (const a of animations) {
-    const s = a.resolvedStart ?? (typeof a.position === "number" ? a.position : 0);
-    const d = a.duration ?? 0;
-    lo = Math.min(lo, s);
-    hi = Math.max(hi, s + d);
-  }
-  if (!Number.isFinite(lo) || !Number.isFinite(hi) || hi <= lo) return null;
-  return { start: lo, duration: hi - lo };
-}
+import { deriveElementTiming } from "./propertyPanelFlatTimingDerivation";
 
 export function FlatTimingRow({
   element,
@@ -32,15 +18,7 @@ export function FlatTimingRow({
   animations?: GsapAnimation[];
   onSetAttribute: (attr: string, value: string) => void | Promise<void>;
 }) {
-  const explicitStart = Number.parseFloat(element.dataAttributes.start ?? "0") || 0;
-  const explicitDuration =
-    Number.parseFloat(
-      element.dataAttributes.duration ?? element.dataAttributes["hf-authored-duration"] ?? "0",
-    ) || 0;
-
-  const derived = explicitDuration > 0 ? null : deriveTimingFromAnimations(animations);
-  const start = derived ? derived.start : explicitStart;
-  const duration = derived ? derived.duration : explicitDuration;
+  const { start, duration, inferred: derived } = deriveElementTiming(element, animations);
   const end = start + duration;
 
   const commitStart = (nextValue: string) => {

packages/studio/src/components/editor/propertyPanelFlatTimingDerivation.test.ts

@@ -0,0 +1,71 @@
+import { describe, expect, it } from "vitest";
+import { deriveElementTiming } from "./propertyPanelFlatTimingDerivation";
+import type { DomEditSelection } from "./domEditingTypes";
+import type { GsapAnimation } from "@hyperframes/core/gsap-parser";
+
+function withDataAttributes(
+  dataAttributes: Record<string, string>,
+): Pick<DomEditSelection, "dataAttributes"> {
+  return { dataAttributes };
+}
+
+describe("deriveElementTiming", () => {
+  it("uses the explicit data-start/data-duration attributes when duration is authored", () => {
+    const result = deriveElementTiming(withDataAttributes({ start: "8", duration: "4" }));
+    expect(result).toEqual({ start: 8, duration: 4, inferred: false });
+  });
+
+  it("infers start/duration from animations when there is no explicit data-duration", () => {
+    const animations = [{ position: 2, duration: 3 } as unknown as GsapAnimation];
+    const result = deriveElementTiming(
+      withDataAttributes({ start: "0", duration: "0" }),
+      animations,
+    );
+    expect(result).toEqual({ start: 2, duration: 3, inferred: true });
+  });
+
+  it("spans the earliest tween start to the latest tween end across multiple animations", () => {
+    const animations = [
+      { position: 1, duration: 2 } as unknown as GsapAnimation, // 1 -> 3
+      { position: 2, duration: 4 } as unknown as GsapAnimation, // 2 -> 6
+    ];
+    const result = deriveElementTiming(withDataAttributes({}), animations);
+    expect(result).toEqual({ start: 1, duration: 5, inferred: true });
+  });
+
+  it("prefers an explicit data-duration over inference even when animations exist", () => {
+    const animations = [{ position: 2, duration: 3 } as unknown as GsapAnimation];
+    const result = deriveElementTiming(
+      withDataAttributes({ start: "0", duration: "10" }),
+      animations,
+    );
+    expect(result).toEqual({ start: 0, duration: 10, inferred: false });
+  });
+
+  it("falls back to hf-authored-duration when data-duration is absent", () => {
+    const result = deriveElementTiming(
+      withDataAttributes({ start: "1", "hf-authored-duration": "6" }),
+    );
+    expect(result).toEqual({ start: 1, duration: 6, inferred: false });
+  });
+
+  it("returns a zero-duration, non-inferred result with no attributes and no animations", () => {
+    const result = deriveElementTiming(withDataAttributes({}));
+    expect(result).toEqual({ start: 0, duration: 0, inferred: false });
+  });
+
+  // This is the exact bug from the whole-plan coherence review: Layout's
+  // keyframe-seek basis must land on the same absolute time that Motion's
+  // Timing row displays as the element's midpoint.
+  it("agrees with a keyframe-percentage seek: 50% lands on the same midpoint the Timing row would show", () => {
+    const animations = [{ position: 2, duration: 3 } as unknown as GsapAnimation];
+    const timing = deriveElementTiming(
+      withDataAttributes({ start: "0", duration: "0" }),
+      animations,
+    );
+    const seekTimeAt50Pct = timing.start + (50 / 100) * timing.duration;
+    const timingRowMidpoint = timing.start + timing.duration / 2;
+    expect(seekTimeAt50Pct).toBe(timingRowMidpoint);
+    expect(seekTimeAt50Pct).toBe(3.5);
+  });
+});

packages/studio/src/components/editor/propertyPanelFlatTimingDerivation.ts

@@ -0,0 +1,59 @@
+import type { GsapAnimation } from "@hyperframes/core/gsap-parser";
+import type { DomEditSelection } from "./domEditingTypes";
+
+/**
+ * The single source of truth for an element's clip start/duration in the flat
+ * inspector. Both the Motion group's Timing row (`FlatTimingRow`) and the
+ * Layout group's keyframe gutter (fed via `elStart`/`elDuration` from
+ * `PropertyPanel.tsx` through `PropertyPanelFlat.tsx`) must derive this the
+ * same way — otherwise a keyframe-percentage seek in Layout lands on a
+ * different absolute time than the range Motion displays for the same
+ * element (found by the Plan 3a+3b whole-plan coherence review).
+ *
+ * Precedence: an explicit `data-duration` (or `data-hf-authored-duration`)
+ * wins outright. Only when neither is present do we infer the range from the
+ * element's own GSAP tweens (earliest tween start → latest tween end).
+ *
+ * Scoped to the FLAT inspector only — the legacy (non-flat) panel keeps its
+ * own, unrelated `elStart`/`elDuration ?? 1` computation in `PropertyPanel.tsx`
+ * untouched.
+ */
+export interface ElementTiming {
+  start: number;
+  duration: number;
+  /** True when duration/start came from `deriveTimingFromAnimations`, not an authored attribute. */
+  inferred: boolean;
+}
+
+function deriveTimingFromAnimations(
+  animations: GsapAnimation[],
+): { start: number; duration: number } | null {
+  let lo = Infinity;
+  let hi = -Infinity;
+  for (const a of animations) {
+    const s = a.resolvedStart ?? (typeof a.position === "number" ? a.position : 0);
+    const d = a.duration ?? 0;
+    lo = Math.min(lo, s);
+    hi = Math.max(hi, s + d);
+  }
+  if (!Number.isFinite(lo) || !Number.isFinite(hi) || hi <= lo) return null;
+  return { start: lo, duration: hi - lo };
+}
+
+export function deriveElementTiming(
+  element: Pick<DomEditSelection, "dataAttributes">,
+  animations: GsapAnimation[] = [],
+): ElementTiming {
+  const explicitStart = Number.parseFloat(element.dataAttributes.start ?? "0") || 0;
+  const explicitDuration =
+    Number.parseFloat(
+      element.dataAttributes.duration ?? element.dataAttributes["hf-authored-duration"] ?? "0",
+    ) || 0;
+
+  const derived = explicitDuration > 0 ? null : deriveTimingFromAnimations(animations);
+  return {
+    start: derived ? derived.start : explicitStart,
+    duration: derived ? derived.duration : explicitDuration,
+    inferred: derived !== null,
+  };
+}