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Yes, 251112-1 is the version I last tested. And I've just confirmed that the problem does not seem to happen on Chrome, but still does happen on Firefox (both on macOS).

I am still seeing the same problem. What did you change? And is there a difference in environment that causes this problem to not happen on your side? In case it matters, I'm using Firefox 144.0.2 (64-bit).

When generating an APNG file, the frames are often out of order. This is because addFrame() in APNGBuilder is an async function, so one should either write another async function to call it with await, or directly handle the returned Promise object and use promise chaining, to guarantee the correct execution order.