BufferedStream.WriteByte had a surprising side effect through .NET 9: the call at its internal capacity boundary also called Flush() on the wrapped stream. The .NET 10 BufferedStream WriteByte behavior removes that implicit flush. Bytes can still move to the underlying stream when the buffer needs room, but that capacity boundary no longer becomes an accidental flush boundary. That distinction matters when a custom stream, protocol adapter, compressor, or test double gives Flush() obse...