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Animations a script creates before the runtime loads land up to a frame off, differently each load

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
typescript
Domain
computer-graphics, frontend

Research direction

Start in packages/core/src/runtime/adapters/waapi.ts, focusing on ensureBaseline and how it records an animation's currentTime. Run the supplied headless Chrome reproduction with window.__player.seek(2) across several page loads, then verify the computed transform consistently reads translateX(50px) for the pre-runtime animation.

Written by the indexing model from the issue text.

Description

What happens

An animation a composition's script creates with el.animate() before the runtime loads lands up to a frame off after a seek, differently on each page load. A 4 s linear 100 px slide seeked to 2 s reads 50 to 51.25 px instead of 50.

The WAAPI adapter records a baseline for each animation the first time it sees it, including the animation's own currentTime at that moment (ensureBaseline in packages/core/src/runtime/adapters/waapi.ts). An animation created before the runtime has already been running on the wall clock for however long loading took, so every later seek is offset by that amount. Animations created after the runtime loads go through the adapter's Element.prototype.animate hook and are anchored where they are created.

Repro

Headless Chrome, the runtime built from main, window.__player.seek(2), then read the computed transform. Load the page several times.

<div data-composition-id="main" data-root="true" data-start="0" data-duration="10" data-width="640" data-height="360">
  <div id="el" class="clip" data-start="0" data-duration="10">el</div>
</div>
<script>
  document.getElementById("el").animate(
    [{ transform: "translateX(0)" }, { transform: "translateX(100px)" }],
    { duration: 4000, easing: "linear", fill: "both" },
  );
</script>
<!-- then the runtime script -->

Expected: translateX(50px) on every load.
Observed: 50 to 51.25 px, varying by load.

Notes

  • Same family as #4555 (CSS animations in shadow roots and on pseudo-elements), which anchors CSS animations at their clip's start. Script-created animations need their own decision about where they start, so they are kept separate.
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