The resampler leftover buffer can contain data from a sink input, data which has been popped from the sink input implementor but not yet pushed to the render_memblockq. That data is not currently taken into account when rewinding. When doing a rewind that affects the resampler, the leftover data is discarded, and that is not compensated in any way, which means that the data is completely lost and there's a skip in the audio.
In practice the leftover buffer is rarely used and even if it is used, it will contain a minimal amount of data, so the user-visible effect of this bug is minor.
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