Decoder only reads X bytes (512? I don’t remember) and if the structure fits into these bytes exactly (and the multiplications of it) the newline leaves hanging there.
I always check for Decoder.More (this drains whitespace) after decoding.
The standard library maintainers recognized this as a problem that there was no “better,” more convenient way to do it and in encoding/json/v2 they have UnmarshalRead for that use case: https://pkg.go.dev/encoding/json/v2#UnmarshalRead
Don't believe there was a more convenient way to achieve the same without writing our own Decoders/Encoders.
https://cs.opensource.google/go/go/+/refs/tags/go1.27.2:src/...
What it won't do looking at the code is handle streamed responses over transfer-encoding:chunked which means that if the other end is doing
json.NewEncoder(responseWriter).Encode(&someStruct)
Turns out that json.Encoder always appends a newline but (as GP said) json.Decoder will not attempt to read it if the JSON is complete at the power-of-two byte boundary (IIRC the buffer doubles for the first few iterations). The extra byte in the pipe when closed then causes reads and writes to go boom. We switched to using json.Marshal because it doesn't output a newline (sadly the encoded form isn't streamed as a result).
I still get twitchy around encoding/json as a result of that experience.
[1]: https://github.com/docker/docker/issues/14203#issuecomment-1...