fix: preserve line feed character references in attribute values

[giaBaoJS]

Fixes #15555

<span title="A&#x0A;B"> currently renders with a space instead of a line break, so there is no way to get a newline into an attribute value using a character reference. 
 works, and title={'A\nB'} works, but the LF entity silently loses the newline.

Why it happens

decode_character_references() already knows whether it is decoding an attribute value — is_attribute_value is threaded in from element.js (true) and text.js (false) — but the flag stops at the call to validate_code() on line 63. validate_code() unconditionally maps code point 10 to 32:

// line feed becomes generic whitespace
if (code === 10) {
	return 32;
}

That is correct for text content, where the line feed would be collapsed along with the surrounding whitespace anyway, but an attribute value is not whitespace-normalized, so the collapse is simply lossy there.

The fix

Pass is_attribute_value down to validate_code() and skip the collapse when it is set. Nothing else changes: every other code point takes the same path it did before, and text content still collapses.

Changes

• 1-parse/utils/html.js: thread is_attribute_value into validate_code() and guard the LF collapse with it
• tests/parser-legacy/samples/convert-entities-line-feed: AST snapshot pinning both halves — <p title="A&#x0A;B">A&#x0A;B</p> parses to data: "A\nB" for the attribute and data: "A B" for the text
• tests/runtime-runes/samples/html-entity-line-feed: the reported case end to end — hex and decimal LF entities on title, an LF entity in a prop passed to a component, and a © control to show ordinary entities still decode

Test plan

pnpm test
#  Test Files  34 passed (34)
#  Tests  7756 passed | 69 skipped (7825)
#  (7753 on main, +3 from the new fixtures)

pnpm lint                    # clean
pnpm --filter svelte check   # clean

Reverting just the html.js change turns both fixtures red:

FAIL packages/svelte/tests/runtime-runes/test.ts > html-entity-line-feed (dom)
FAIL packages/svelte/tests/runtime-runes/test.ts > html-entity-line-feed (hydrate)
AssertionError: expected 'A B' to equal 'A\nB'

FAIL packages/svelte/tests/parser-legacy/test.ts > convert-entities-line-feed
-  "data": "A\nB",
+  "data": "A B",

The parser snapshot is deliberately the one that guards against over-fixing: in that diff only the attribute data moves, the text data stays "A B". Replacing the guard with a wholesale removal of the LF rule flips the other half red instead, on the text node, so the fixture pins the behaviour in both directions.

One thing I noticed while writing the runtime test but did not touch, since it is unrelated and predates this change: for text content the client backend inlines the raw source into the template ($.from_html(\A B`)) and lets the browser decode it, so the DOM ends up with a real newline, while SSR emits the collapsed A B`. That client/SSR divergence is identical before and after this PR. Happy to open a separate issue for it if it is worth tracking.

[pkg-svelte-dev]

Install the latest version of svelte from 5a40c98:

pnpm add https://pkg.svelte.dev/svelte/c/5a40c98d5d5e70e3134d5afa892a565bf17cd2e0

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18691

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5a40c98

The changes in this PR will be included in the next version bump.

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