The reasoning behind https://hg.mozilla.org/mozilla-central/rev/f594e6f00208 was incorrect because UpdateReceivedNewData() can [allow](https://searchfox.org/mozilla-central/rev/48caa4719ed2c75a2937bcfea9e7326e87846984/dom/media/MediaFormatReader.cpp#1914) Update to proceed when `mSeekRequest.Exists()`. That changeset (f594e6f00208) should be backed out.
Bug 1941164 Comment 2 Edit History
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The reasoning behind https://hg.mozilla.org/mozilla-central/rev/f594e6f00208 was not quite accurate because UpdateReceivedNewData() can [allow](https://searchfox.org/mozilla-central/rev/48caa4719ed2c75a2937bcfea9e7326e87846984/dom/media/MediaFormatReader.cpp#1914) Update to proceed when `mSeekRequest.Exists()`. However, Update() will [return early](https://searchfox.org/mozilla-central/rev/48caa4719ed2c75a2937bcfea9e7326e87846984/dom/media/MediaFormatReader.cpp#2351,2353) anyway when `decoder.mSeekRequest.Exists()`. Still trying to understand how this state is reached.