`@defer` support with `accept: multipart/mixed` header rather than `accept: text/event-stream`

What happened?

I'm using Apollo GraphQL on the frontend, which automatically passes accept: multipart/mixed as header when the query contains a @defer directive. Unfortunately this causes the gqlgen server to expire the context and actually not return the deferred fields.

If I were to use accept: text/event-stream as header, the gqlgen server actually handles the context correctly and returns the deferred data.

Example:

 ▲ ~/work/gqlgen-todos curl 'http://localhost:8080/query' -H 'Connection: keep-alive' -H 'accept: text/event-stream' -H 'content-type: application/json; charset=utf-8' --data-raw '{"query":"query test { todos { ... on Todo @defer { user { id } } } }"}'
:

event: next
data: {"data":{"todos":[{"user":null},{"user":null}]},"hasNext":true}

event: next
data: {"data":{"user":{"id":"1"}},"path":["todos",1],"hasNext":true}

event: next
data: {"data":{"user":{"id":"1"}},"path":["todos",0],"hasNext":false}

event: complete

 ▲ ~/work/gqlgen-todos curl 'http://localhost:8080/query' -H 'Connection: keep-alive' -H 'accept: multipart/mixed' -H 'content-type: application/json; charset=utf-8' --data-raw '{"query":"query test { todos { ... on Todo @defer { user { id } } } }"}'
{"data":{"todos":[{"user":null},{"user":null}]},"hasNext":true}%

What did you expect?

Context to not expire and subsequent multipart responses returned to the frontend.

Is there an additional configuration that needs to be set up in the gqlgen server or in Gin that I might be missing?

Minimal graphql.schema and models to reproduce

Example repo based off the official documentation with gin: https://github.com/giulio-opal/defer-gqlgen-todo-example

It also contains a custom playground (GraphiQL) based off https://github.com/99designs/gqlgen/issues/2771#issuecomment-1694692940

versions

go run github.com/99designs/gqlgen version? v0.17.49+
go version? 1.22.0

99designs / gqlgen