diff --git a/internal/interaction-harness/README.md b/internal/interaction-harness/README.md
index 0166202d9ec44..4cce8060a77da 100644
--- a/internal/interaction-harness/README.md
+++ b/internal/interaction-harness/README.md
@@ -278,7 +278,11 @@ a just-started compositor animation late (a sheet's release held on screen for
 late on a loaded runner, too late to wait on before blocking. Only a hold that
 starts before the block, on two frames at least, at the position the page last
 set before the block is excused. Pass that position as `setBeforeBlock`, such
-as the drag's last position at the release. An animation frozen at the block's
+as the drag's last position at the release. Without it, a block with `during`
+takes the position on the cast's last frame before the caught animation's
+start. Chromium on a loaded runner presents a just-started compositor
+animation late too: a slider's coast first showed 25–111 ms after its start.
+An animation frozen at the block's
 start holds a position of its own, and is judged as a stall. Every frame after
 its first change is judged, so a freeze later in the block still fails.
 An element off screen before the block, such as a card that enters from an
diff --git a/internal/interaction-harness/src/busy.control.test.ts b/internal/interaction-harness/src/busy.control.test.ts
index a17129f332db2..467a408cfb676 100644
--- a/internal/interaction-harness/src/busy.control.test.ts
+++ b/internal/interaction-harness/src/busy.control.test.ts
@@ -1048,4 +1048,142 @@ describe('a hold that starts before the block', () => {
     expect(proof.verdict.pass, proof.summary).toBe(false);
     expect(proof.onScreenFromMs).toBeNull();
   });
+
+  // A window that caught the animation (`blockMainThread`'s `during`) knows
+  // its start. The position on the cast's last frame before that start then
+  // stands for the position the page set.
+  const caught = {
+    ...BLOCK,
+    animation: {from: holdFrom, to: BLOCK.to + 500, holds: true},
+  };
+
+  it('takes the position from the last frame before the caught animation starts when no position is given', () => {
+    const proof = busyWindowProof(sheet(), caught, {axis: 'y', control});
+    const named = busyWindowProof(sheet(), caught, {
+      axis: 'y',
+      control,
+      setBeforeBlock: released,
+    });
+    expect(proof.verdict.failures, proof.summary).toEqual([]);
+    expect(proof.onScreenFromMs).toBeGreaterThanOrEqual(100);
+    expect(proof.summary).toBe(named.summary);
+  });
+
+  it("keeps the position the page set over the caught animation's start", () => {
+    const proof = busyWindowProof(sheet(), caught, {
+      axis: 'y',
+      control,
+      setBeforeBlock: released + 6,
+    });
+    expect(proof.verdict.pass, proof.summary).toBe(false);
+    expect(proof.onScreenFromMs).toBeNull();
+  });
+
+  it('still fails a main-thread animation held at its start until after the block', () => {
+    let y = 0;
+    const frames = moving().map(f => {
+      if (f.t < holdFrom || f.t >= BLOCK.to + 50) {
+        y += 3;
+      }
+      return {...f, box: {...f.box!, y}} as TrackedFrame;
+    });
+    const proof = busyWindowProof(frames, caught, {axis: 'y', control});
+    expect(proof.verdict.pass, proof.summary).toBe(false);
+    expect(proof.onScreenFromMs).toBeNull();
+  });
+
+  it('does not excuse a hold at a position the animation moved to after its start', () => {
+    // Moving for 60 ms after the caught animation's start, then held there
+    // until 100 ms into the block, then moving on every frame.
+    let y = 0;
+    const frames = moving().map(f => {
+      if (f.t < holdFrom + 60 || f.t >= holdTo) {
+        y += 3;
+      }
+      return {...f, box: {...f.box!, y}} as TrackedFrame;
+    });
+    const proof = busyWindowProof(frames, caught, {axis: 'y', control});
+    expect(proof.verdict.pass, proof.summary).toBe(false);
+    expect(proof.onScreenFromMs).toBeNull();
+  });
+
+  it("judges a recorded slider coast the same with the caught animation's start as with the position the page set", () => {
+    // Recorded in Chromium on a loaded runner: SliderTouch's coast through a
+    // 400 ms block, with the busy control. Each row is [ms from the block's
+    // start, thumb x, control x] on one cast frame. The thumb holds at its
+    // release position, 163 px, from before the coast's start. Here the hold
+    // is extended through the second judged frame, as a late hand-off can be.
+    const recorded: ReadonlyArray<readonly [number, number, number]> = [
+      [-222.1, 156, 130],
+      [-188.6, 156, 150],
+      [-179, 156, 160],
+      [-153.7, 163, 180],
+      [-144.6, 163, 190],
+      [-122.4, 163, 200],
+      [-86.2, 163, 210],
+      [-85.6, 163, 220],
+      [-45.2, 163, 240],
+      [-29.8, 163, 230],
+      [-24.1, 163, 220],
+      [15.2, 206, 190],
+      [42.1, 214, 180],
+      [49.2, 220, 170],
+      [66.8, 225, 160],
+      [93.7, 229, 150],
+      [135.3, 232, 140],
+      [139, 236, 130],
+      [171.4, 241, 100],
+      [186.5, 243, 90],
+      [210.6, 244, 80],
+      [231.8, 245, 60],
+      [259.7, 246, 50],
+      [279.9, 246, 40],
+      [287.7, 247, 30],
+      [318.4, 247, 20],
+    ];
+    const from = 10_000;
+    const coastStart = from - 74.7;
+    const holdUntil = from + 49.2;
+    const thumb = recorded.map(
+      ([t, x]) =>
+        ({
+          t: from + t,
+          box: {
+            x: from + t > coastStart && from + t <= holdUntil ? 163 : x,
+            y: 0,
+            width: 28,
+            height: 28,
+          },
+          pixels: 400,
+          step: {x: 1, y: 1},
+        }) as TrackedFrame,
+    );
+    const slider = recorded.map(
+      ([t, , x]) =>
+        ({
+          t: from + t,
+          box: {x, y: 0, width: 12, height: 12},
+          pixels: 100,
+        }) as TrackedFrame,
+    );
+    const window = {
+      from,
+      to: from + 400,
+      animation: {from: coastStart, to: coastStart + 564.3, holds: true},
+    };
+    const options = {
+      axis: 'x',
+      settledWithinSteps: 2,
+      control: slider,
+    } as const;
+    const byStart = busyWindowProof(thumb, window, options);
+    const named = busyWindowProof(thumb, window, {
+      ...options,
+      setBeforeBlock: 163,
+    });
+    const plain = busyWindowProof(thumb, {from, to: from + 400}, options);
+    expect(byStart.verdict.failures, byStart.summary).toEqual([]);
+    expect(byStart.summary).toBe(named.summary);
+    expect(plain.verdict.pass, plain.summary).toBe(false);
+  });
 });
diff --git a/internal/interaction-harness/src/busy.ts b/internal/interaction-harness/src/busy.ts
index ff21f0e52fac7..614813d146bed 100644
--- a/internal/interaction-harness/src/busy.ts
+++ b/internal/interaction-harness/src/busy.ts
@@ -1797,10 +1797,13 @@ function motionStepPx(values: readonly number[]): number {
  * at the first presented change inside it: WebKit presents a just-started
  * compositor animation late (a sheet's release held on screen for 130 to
  * 300 ms while its timeline ran, and the screencast reaches Node about 0.7 s
- * late on a loaded runner, too late to wait on before blocking). Only a hold
+ * late on a loaded runner, too late to wait on before blocking). Chromium on
+ * a loaded runner does too. A slider's coast first showed 25–111 ms after
+ * its start (median 57 ms), with a catch-up on that first frame. Only a hold
  * at `setBeforeBlock`, the value the page itself last set before the block,
- * is excused. Every frame after that first change is judged, so a freeze
- * later in the block fails.
+ * is excused. Without it, a window with a caught animation takes the value on
+ * the cast's last frame before that animation's start. Every frame after that
+ * first change is judged, so a freeze later in the block fails.
  * An element off screen before the judged part is judged from its first
  * position inside the part, as a card that enters from an edge. The cast's
  * frames before the part must show the screen without it. A main-thread
@@ -2154,9 +2157,11 @@ function judgeBusyWindow(
      * coordinates: where the box sits at rest, plus the offset the page wrote.
      * A hold that starts before the block and ends at its first presented
      * change can be a late hand-off. It is one only when it holds this value,
-     * where the animation starts. Without it, or when the hold shows another
-     * value, the hold is judged as a stall. An animation can freeze at the
-     * block's start.
+     * where the animation starts. Without it, a window that caught an
+     * animation (`blockMainThread`'s `during`) takes the value on the cast's
+     * last frame before that animation's start. When the hold shows another
+     * value, or no value is known, the hold is judged as a stall. An
+     * animation can freeze at the block's start.
      */
     setBeforeBlock?: number;
     /**
@@ -2383,13 +2388,23 @@ function judgeBusyWindow(
   // up to a step off. A cast with no step is read within a pixel, and a
   // sub-pixel read within half a pixel.
   const matchPx = subpixelStepPx != null ? 0.5 : (readStepPx ?? 1);
+  // Where the animation starts: the value the page set (`setBeforeBlock`).
+  // Without it, the caught animation's start dates it: the value on the
+  // cast's last frame before that start. Unlike the progress replay's
+  // anchor, a frame that shows no element is not skipped. An earlier value
+  // can be stale, so the hold then gets no excuse.
+  const preStart =
+    setBeforeBlock ??
+    (heldCurve == null
+      ? undefined
+      : [...tracked].reverse().find(f => f.t < heldCurve.from)?.box?.[axis]);
   const heldFromBeforeBlock =
-    setBeforeBlock != null &&
+    preStart != null &&
     lead.length > 0 &&
     holdStart > 0 &&
     prior[holdStart].t < window.from &&
     heldFrames >= 2 &&
-    Math.abs(lead[lead.length - 1] - setBeforeBlock) <= matchPx;
+    Math.abs(lead[lead.length - 1] - preStart) <= matchPx;
   // Off screen before the judged part, as a card that enters from an edge:
   // the cast's frames there show the screen without it. Its first position
   // inside the part is then its on-screen start, and judging starts there.