In SourceBufferParserWebM::StreamingVectorReader::Read the number of bytes actually read should be returned to the caller via the out parameter numActuallyRead. However, this only returns the number of bytes read on currentSegment ; it's not incremented after each loop.
<rdar://problem/79750832>
This will be resolved in 226069. *** This bug has been marked as a duplicate of bug 226069 ***