diff --git a/packages/core/src/runtime/init.rootLength.test.ts b/packages/core/src/runtime/init.rootLength.test.ts
new file mode 100644
index 0000000000..0dc084c4ba
--- /dev/null
+++ b/packages/core/src/runtime/init.rootLength.test.ts
@@ -0,0 +1,64 @@
+import gsap from "gsap";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { initSandboxRuntimeModular } from "./init";
+import type { RuntimeTimelineLike } from "./types";
+import { resetRuntimeFixtureDom } from "./runtimeSeekFixture.test-helpers";
+
+// A render seek must draw each scene once, at its own time, however many scenes follow the playhead.
+describe("runtime root length across render seeks", () => {
+  beforeEach(() => {
+    resetRuntimeFixtureDom();
+    window.gsap = gsap as unknown as typeof window.gsap;
+  });
+
+  afterEach(() => {
+    window.__hfRuntimeTeardown?.();
+    delete window.__player;
+    delete window.__playerReady;
+    delete window.__hf;
+  });
+
+  /** `sceneCount` five-second scenes back to back; returns the local times scene 1 is drawn at. */
+  function load(sceneCount: number) {
+    const hosts = Array.from(
+      { length: sceneCount },
+      (_, i) => `<div data-composition-id="s${i}" data-start="${i * 5}" data-duration="5"></div>`,
+    ).join("");
+    document.body.innerHTML = `<div data-composition-id="main" data-start="0" data-duration="${sceneCount * 5}">${hosts}</div>`;
+    const timelines: Record<string, RuntimeTimelineLike> = {
+      main: gsap.timeline({ paused: true }) as unknown as RuntimeTimelineLike,
+    };
+    const drawnAt: number[] = [];
+    for (let i = 0; i < sceneCount; i++) {
+      const target = { v: 0, w: 0 };
+      const scene = gsap
+        .timeline({ paused: true })
+        .to(target, { v: 1, duration: 5, ease: "none" }, 0)
+        .to(target, { w: 1, duration: 1 }, 0)
+        // Overwrites the tween above on its first render, which marks the root's length stale.
+        .to(target, { w: 2, duration: 1, overwrite: "auto" }, 0);
+      if (i === 1)
+        scene.eventCallback("onUpdate", () => drawnAt.push(Number(scene.time().toFixed(4))));
+      timelines[`s${i}`] = scene as unknown as RuntimeTimelineLike;
+    }
+    window.__timelines = timelines;
+    initSandboxRuntimeModular();
+    return drawnAt;
+  }
+
+  function renderFrames(from: number, to: number) {
+    for (let frame = from; frame <= to; frame++) window.__player?.renderSeek(frame / 30);
+  }
+
+  it("draws a scene at its own time when no scene follows it", () => {
+    const drawnAt = load(2);
+    renderFrames(210, 214);
+    expect(drawnAt).toEqual([2, 2.0333, 2.0667, 2.1, 2.1333]);
+  });
+
+  it("draws a scene at its own time when a later scene first renders during the seek", () => {
+    const drawnAt = load(3);
+    renderFrames(210, 214);
+    expect(drawnAt).toEqual([2, 2.0333, 2.0667, 2.1, 2.1333]);
+  });
+});
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index fab9328bc3..ac9ca35880 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -4638,6 +4638,14 @@ export function initSandboxRuntimeModular(): void {
         swallow("runtime.init.activateSiblings", err);
       }
     }
+    if (rearmed.length === 0) return rearmed;
+    // GSAP leaves paused scenes out of the root's length and play() keeps that stale length.
+    try {
+      masterTimeline.shiftChildren?.(0);
+      masterTimeline.totalDuration?.();
+    } catch (err) {
+      swallow("runtime.init.remeasureRoot", err);
+    }
     return rearmed;
   };
 
diff --git a/packages/core/src/runtime/types.ts b/packages/core/src/runtime/types.ts
index fe70857f01..35ed38aea7 100644
--- a/packages/core/src/runtime/types.ts
+++ b/packages/core/src/runtime/types.ts
@@ -317,6 +317,7 @@ export type RuntimeTimelineLike = {
   seek: (timeSeconds?: number, suppressEvents?: boolean) => unknown;
   totalTime?: (timeSeconds?: number, suppressEvents?: boolean) => unknown;
   totalDuration?: () => number;
+  shiftChildren?: (amount: number) => unknown;
   progress?: (value?: number, suppressEvents?: boolean) => unknown;
   time: () => number;
   duration: () => number;