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When selecting progressive and HLS tracks, we currently check if we can either seek in the existing samples or are starting from the beginning of the stream. In both cases, we don't need to reload the stream and can continue reading. Seeking to the beginning isn't side-effect free though because it also sets the startTimeUs to zero. Whether a seek to the beginning is successful also depends on whether we already loaded a sample or not. This mean that the startTimeUs value is set (or not set) based on the speed of the Loader thread even for the same input starting from the beginning. This race condition means the actual samples we write to the stream may differ if the stream has initial samples with negative timestamps. We can avoid this race condition by checking if the queue is empty first, so that we only call seekTo if really needed. PiperOrigin-RevId: 546010951