fix(formatter_core): leave blank frontmatter without dispatcher (#27359)

Always leave FM (in any form) as-is when `embeddedLanguageFormatting: off` (or Rust CLI).

apps/oxfmt/test/api/front_matter.test.ts

@@ -68,6 +68,13 @@ describe("CSS front matter", () => {
     });
     expect(result.errors).toStrictEqual([]);
     expect(result.code).toBe("---\ntitle:   Home\n---\n\nb {\n}\n");
+
+    // Even an empty block is not normalized
+    const empty = await format("a.css", "---\n\n---\nb {}\n", {
+      embeddedLanguageFormatting: "off",
+    });
+    expect(empty.errors).toStrictEqual([]);
+    expect(empty.code).toBe("---\n\n---\n\nb {\n}\n");
   });
 
   it("keeps a physical BOM at byte 0, before the block", async () => {

crates/oxc_formatter_core/src/envelope/mod.rs

@@ -18,13 +18,13 @@ use crate::{
 
 /// Writes a front matter block.
 ///
-/// The body dispatches through the session when its resolved language is in `embeddable_languages`,
-/// the whole block stays verbatim otherwise.
+/// The body dispatches through the session when a dispatcher is installed
+/// and its resolved language is in `embeddable_languages`, the whole block stays verbatim otherwise.
 /// Membership means "gets the frame treatment", not "gets formatted":
 /// a member language without a serving formatter keeps its body verbatim (`PreserveOriginal`),
-/// but its EMPTY block still normalizes through the frame.
-/// Never fails; any refusal (`PreserveOriginal`, operational error, non-embeddable language) keeps the block's bytes as-is
-/// while the host document still formats.
+/// but its EMPTY block still normalizes through the frame when a dispatcher is installed.
+/// Never fails; any refusal (`PreserveOriginal`, operational error, no dispatcher, non-embeddable language)
+/// keeps the block's bytes as-is while the host document still formats.
 ///
 /// The composed shape (Prettier `embed.js` + its css/markdown printers):
 /// opening delimiter with the explicit language re-emitted (`---yaml`),
@@ -36,7 +36,7 @@ pub fn write_front_matter<'a, C: FormatContext>(
     f: &mut Formatter<'_, 'a, C>,
 ) {
     let language = fm.language();
-    if embeddable_languages.contains(&language) {
+    if f.session().has_dispatcher() && embeddable_languages.contains(&language) {
         let body = fm.value.trim();
         if body.is_empty() {
             write_frame(fm, None, f);

crates/oxc_formatter_css/src/format.rs

@@ -31,7 +31,7 @@ pub fn format<'a>(
     options: CssFormatOptions,
 ) -> Result<Formatted<'a, CssFormatContext<'a>>, OxcDiagnostic> {
     // NOTE: this wrapper labels the run `PhysicalFile` with NO services:
-    // front matter is detected but its body degrades to verbatim (`PreserveOriginal`),
+    // front matter is detected but kept verbatim (no dispatcher),
     // and `@apply` Tailwind classes print unsorted.
     // Hosts that want them use `format_with_session` with the services installed.
     format_with_session(

crates/oxc_formatter_css/tests/conformance.rs

@@ -26,8 +26,9 @@ const CSS: ConformanceConfig = ConformanceConfig {
     ignore: &[
         // postcss-conditionals (archived: https://github.com/andyjansson/postcss-conditionals).
         "css/atrule/if-else.css",
-        // YAML frontmatter
+        // YAML frontmatter, needs the dispatcher this harness doesn't install
         "css/yaml/dirty.css",
+        "css/yaml/empty_newlines.css",
         // range formatting / IDE cursor, not whole-file formatting
         "css/range/",
         "css/cursor/",

crates/oxc_formatter_css/tests/snapshots/conformance__prettier-css.snap

@@ -2,7 +2,7 @@
 source: crates/oxc_formatter_css/tests/conformance.rs
 expression: report
 ---
-css compatibility: 142/151 (94.04%), 13 files skipped
+css compatibility: 141/150 (94.00%), 13 files skipped
 
 # Failed