fix(sse): handle multi-line messages

[BSteffaniak]

Fixes an issue where SSE messages containing newlines in the data field would be incorrectly parsed by the client.

Per the SSE spec, multi-line data must be sent as multiple data: lines that the EventSource parser concatenates. Previously, MSW sent the data as a single data: field, which caused messages containing \n\n to be truncated (since \n\n terminates an SSE message).

Before:
data:line1\n\nline2
Client receives: "line1" (truncated)

After:
data:line1
data:
data:line2

Client receives: "line1\n\nline2" (correct)

The fix handles all SSE line terminators per spec: LF (\n), CR (\r), and CRLF (\r\n).

Let me know your thoughts. Thanks!

[kettanaito]

Hi, @BSteffaniak. Thank you for fixing this! And an extra thanks for those test cases.

I've pushed a change related to how the message data is processed, making that processing conditional (no need to fallback to '' to then split via RegExp into an empty array).

Could you please share a link to the paragraph section that outlines how line terminators have to be handled in the SSE spec? We could leave it in the implementation for future references. Thanks.

[BSteffaniak]

Here are some references in the spec:

https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation

here's also some MDN docs around it too (a bit easier to read):

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#data

[BSteffaniak]

This is also relevant: https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream

[kettanaito]

Thanks! This is great.

[kettanaito]

Released: v2.12.5 🎉

This has been released in v2.12.5.

• 📄 Release notes
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Get these changes by running the following command:

npm i msw@latest

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