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test(Lightbox): the busy zoom proof judges the photo's top edge #10648

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Merged
cixzhang merged 2 commits into from
Nov 8, 2026
Merged

test(Lightbox): the busy zoom proof judges the photo's top edge #10648

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test(Lightbox): the busy zoom proof judges the photo's top edge
vjeux Nov 8, 2026
569ba49
test(Lightbox): the busy zoom proof spells out its per-frame check
vjeux Nov 8, 2026
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37 changes: 27 additions & 10 deletions packages/core/src/Lightbox/__tests__/LightboxZoom.interaction.browser.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1081,8 +1081,8 @@ test('the double-tap zoom keeps advancing every frame while the main thread is b
const tracked = await trackMarker(page, castFrames);
const control = await trackMarker(page, castFrames, {rgb: CONTROL_RGB});
const proof = busyWindowProof(tracked, block, {
axis: 'height',
settledWithinSteps: ZOOM_SETTLED_STEPS,
axis: ZOOM_AXIS,
settledWithin: ZOOM_SETTLED_PX,
control,
});
await writeEvidence(
Expand Down Expand Up @@ -1123,6 +1123,12 @@ test('the double-tap zoom keeps advancing every frame while the main thread is b
// animation presents fewer frames there, the control's too. The photo keeps
// pace with the control, and a main-thread animation would not.
expect(proof.verdict.failures).toEqual([]);
// The verdict, spelled out. The top edge takes a new position on every
// frame the control moves, with none excused as under a cast step. The
// control moves on at least the verdict's floor of four frames.
expect(proof.control?.movedWith).toBe(proof.control?.moved);
expect(proof.control?.belowStep).toBe(0);
expect(proof.control?.moved).toBeGreaterThanOrEqual(4);
expect(proof.monotonic).toBe(true);
});

Expand All @@ -1138,13 +1144,24 @@ const ZOOM_TEMPO = 5;
const ZOOM_BLOCK_MS = 2500;

/**
* Where the busy zoom proof stops judging, in cast steps from the zoom's
* end. Near the end, each 60 Hz frame moves the spring about 0.042 of what
* remains (2π / 2.5 s, critically damped). So within about 23.9 cast steps
* of its end a frame moves less than one step, and the cast shows plateaus
* there.
* The photo's coordinate the busy zoom proof judges: its top edge. The zoom
* grows the photo at both edges, and the cast reads each edge in whole
* steps. So near the end the height can show no change for a frame while it
* grows almost two steps a frame. The proof's step rules expect one edge's
* travel, so the proof judges the top edge.
*/
const ZOOM_SETTLED_STEPS = 24;
const ZOOM_AXIS = 'y';

/**
* Where the busy zoom proof stops judging, in px of the top edge from its
* end. There the edge's move between two cast frames falls under one cast
* step. Near the end, each 60 Hz frame moves the spring about 0.042 of what
* remains (2π / 2.5 s, critically damped). Chromium casts every frame in 1 px
* steps, so its moves fall under a step 23.9 px from the end. Blocked, WebKit
Comment on lines +1158 to +1160

@astracat-bot astracat-bot Bot Nov 8, 2026

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Non-blocking. For a critically damped spring from rest, what remains is A(1 + ωt)e^(-ωt), so a frame moves ωΔt · ωt/(1 + ωt) of it. That is about 0.032 at 24 px, not 0.042, which is the rate only as t grows without bound. The recorded top edge fits that curve to within 5 px in total. The move drops under 1 px at about 30.5 px from rest. The last ~4 judged frames move 0.86–0.98 px. In every run here, rounding still shows a new position on each of them, and the first held frame (y = 25) comes one frame after the cut. With expect(proof.control?.belowStep).toBe(0) that one frame is the whole margin. Replaying the same runs with the cast ending at rest (y = 0) fails the new assertion while the verdict passes. A 32 px bound, or settledAt read at rest, keeps belowStep 0 and still judges about 1.3 s.

* casts a frame only every 116 ms or more, seven 60 Hz frames, in 2 px steps.
* Its moves stay over a step until about 8 px from the end, inside 24 px.
*/
const ZOOM_SETTLED_PX = 24;

/** Each animated property's curve length, and whether it still runs. */
function curvesByProperty(runs: ReadonlyArray<ActiveAnimation>) {
Expand Down Expand Up @@ -1220,8 +1237,8 @@ for (const pace of ['main-thread', 'frozen'] as const) {
let failures: ReadonlyArray<string>;
try {
failures = busyWindowProof(tracked, block, {
axis: 'height',
settledWithinSteps: ZOOM_SETTLED_STEPS,
axis: ZOOM_AXIS,
settledWithin: ZOOM_SETTLED_PX,
control,
}).verdict.failures;
} catch (error) {
Expand Down
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