This is a really shonky characteristic of MP3. I can accept that you might need empty space at the first and last frames to get your waveform right, but I'm shocked that the format doesn't allow for "only the first/last x resultant samples of this frame should be played". Maybe the format does allow it, and nobody implements it?

I (obviously from the above comments) have no actual knowledge of the spec. Is there anyone here who knows?

Richard.