HTMX4 SSE parser trims all leading whitespace instead of a single space

[antonlorani]

The SSE parser in htmx 4 currently removes all leading whitespace from field values. According to the SSE spec, only one leading space may be removed, and only if it exists.

Relevant code:

htmx/src/htmx.js

Line 719 in 4403f8e

let value = line.slice(colonIndex + 1).trimStart();

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

Collect the characters on the line after the first U+003A COLON character (:), and let value be that string. If value starts with a U+0020 SPACE character, remove it from value.

Example

Given this SSE payload:

id: 45
event:  message
data:  Hello, World!

Current behavior:

id = "45"
event = "message"
data = "Hello, World!"

Expected behavior (per spec):

id = "45"
event = " message"
data = " Hello, World!"