fix(comark): only complete unclosed frontmatter while streaming

packages/comark/SPEC/COMARK/frontmatter-unclosed-is-thematic-break.md

@@ -0,0 +1,43 @@
+## Input
+
+```md
+---
+# Heading
+```
+
+## AST
+
+```json
+{
+  "frontmatter": {},
+  "meta": {},
+  "nodes": [
+    [
+      "hr",
+      {}
+    ],
+    [
+      "h1",
+      {
+        "id": "heading"
+      },
+      "Heading"
+    ]
+  ]
+}
+```
+
+## HTML
+
+```html
+<hr />
+<h1 id="heading">Heading</h1>
+```
+
+## Markdown
+
+```md
+---
+
+# Heading
+```

packages/comark/SPEC/common-mark/horizontal-rule-bare.md

@@ -0,0 +1,32 @@
+## Input
+
+```md
+---
+```
+
+## AST
+
+```json
+{
+  "frontmatter": {},
+  "meta": {},
+  "nodes": [
+    [
+      "hr",
+      {}
+    ]
+  ]
+}
+```
+
+## HTML
+
+```html
+<hr />
+```
+
+## Markdown
+
+```md
+---
+```

packages/comark/src/internal/parse/auto-close/index.ts

@@ -10,16 +10,21 @@ import { closeTables } from './table.ts'
  * Processes markdown in O(n) time by scanning character-by-character
  *
  * @param markdown - The markdown content to auto-close
+ * @param options - `frontmatter` completes an unclosed leading frontmatter block.
  * @returns The markdown with unclosed syntax closed
  */
-export function autoCloseMarkdown(markdown: string): string {
+export function autoCloseMarkdown(markdown: string, options: { frontmatter?: boolean } = {}): string {
   if (!markdown || markdown === '') return markdown
 
   const lines = markdown.split('\n')
   const n = lines.length
 
   // Single linear pass to collect document state
   let inFrontmatter = false
+  // Whether the open frontmatter block has received any content. Empty
+  // frontmatter is not valid (`parseFrontmatter` ignores it), so a lone `---`
+  // is a thematic break, not an unclosed frontmatter block to complete.
+  let frontmatterHasContent = false
   let inBlockMath = false
   let tableStart = -1
   // Tag name when inside a raw-text HTML element (`<style>`, `<script>`,
@@ -57,12 +62,13 @@ export function autoCloseMarkdown(markdown: string): string {
     }
 
     // Frontmatter: only starts at document line 0
-    if (idx === 0 && trimmed === '---') {
+    if (idx === 0 && options.frontmatter && trimmed === '---') {
       inFrontmatter = true
       continue
     }
     if (inFrontmatter) {
       if (trimmed === '---') inFrontmatter = false
+      else if (trimmed !== '') frontmatterHasContent = true
       continue
     }
 
@@ -152,8 +158,8 @@ export function autoCloseMarkdown(markdown: string): string {
     result = closeTables(result)
   }
 
-  // Close unclosed frontmatter
-  if (inFrontmatter) {
+  // Complete an unclosed frontmatter block only when `frontmatter` is enabled,
+  if (inFrontmatter && frontmatterHasContent) {
     const lastTrimmed = lines[lastIdx].trim()
     if (lastTrimmed === '-' || lastTrimmed === '--') {
       result += '-'.repeat(3 - lastTrimmed.length)

packages/comark/src/parse.ts

@@ -116,7 +116,7 @@ export function createParse<const TPlugins extends readonly ComarkPlugin<any, an
     }
 
     if (autoClose) {
-      state.markdown = autoCloseMarkdown(state.markdown)
+      state.markdown = autoCloseMarkdown(state.markdown, { frontmatter: opts.streaming })
     }
 
     for (const plugin of options.plugins || []) {

packages/comark/test/auto-close.test.ts

@@ -497,30 +497,41 @@ describe('frontmatter', () => {
     const expected = '---\ntitle: Test\n---'
     expect(autoCloseMarkdown(input)).toBe(expected)
   })
-  it('should handle frontmatter partial', () => {
+  // Partial frontmatter is only completed when `frontmatter: true` — for a
+  // complete document a leading `---` with no closing delimiter is a thematic
+  // break, so completing it would swallow the body (see the default cases below).
+  it('should complete partial frontmatter when frontmatter option is enabled', () => {
     const input = '---\ntitle: Test'
     const expected = '---\ntitle: Test\n---'
-    expect(autoCloseMarkdown(input)).toBe(expected)
+    expect(autoCloseMarkdown(input, { frontmatter: true })).toBe(expected)
   })
-  it('should handle frontmatter partial', () => {
+  it('should complete a partial closing delimiter (-) when frontmatter option is enabled', () => {
     const input = '---\ntitle: Test\n-'
     const expected = '---\ntitle: Test\n---'
-    expect(autoCloseMarkdown(input)).toBe(expected)
+    expect(autoCloseMarkdown(input, { frontmatter: true })).toBe(expected)
   })
-  it('should handle frontmatter partial', () => {
+  it('should complete a partial closing delimiter (--) when frontmatter option is enabled', () => {
     const input = '---\ntitle: Test\n--'
     const expected = '---\ntitle: Test\n---'
-    expect(autoCloseMarkdown(input)).toBe(expected)
+    expect(autoCloseMarkdown(input, { frontmatter: true })).toBe(expected)
   })
-  it('should handle frontmatter partial 2', () => {
+  it('should complete partial frontmatter with an empty value when frontmatter option is enabled', () => {
     const input = '---\ntitle: '
     const expected = '---\ntitle: \n---'
-    expect(autoCloseMarkdown(input)).toBe(expected)
+    expect(autoCloseMarkdown(input, { frontmatter: true })).toBe(expected)
   })
-  it('should handle frontmatter partial 3', () => {
+  it('should complete partial frontmatter with a trailing newline when frontmatter option is enabled', () => {
     const input = '---\ntitle: Test\n'
     const expected = '---\ntitle: Test\n---'
-    expect(autoCloseMarkdown(input)).toBe(expected)
+    expect(autoCloseMarkdown(input, { frontmatter: true })).toBe(expected)
+  })
+
+  it('should not complete unclosed frontmatter by default', () => {
+    // Default (`frontmatter: false`): a leading `---` with content but no close
+    // is a thematic break followed by body — completing it would swallow the body.
+    const input = '---\ntitle: Test'
+    expect(autoCloseMarkdown(input)).toBe(input)
+    expect(autoCloseMarkdown('---\n# Heading')).toBe('---\n# Heading')
   })
 
   it('should handle frontmatter with content after', () => {
@@ -541,9 +552,17 @@ describe('frontmatter', () => {
     expect(autoCloseMarkdown(input)).toBe(expected)
   })
 
-  it('should handle just opening ---', () => {
+  it('should not complete a lone --- (thematic break, not frontmatter)', () => {
+    // A lone `---` has no frontmatter content; completing it to `---\n---`
+    // would parse as two thematic breaks (#268).
     const input = '---'
-    const expected = '---\n---'
+    const expected = '---'
+    expect(autoCloseMarkdown(input)).toBe(expected)
+  })
+
+  it('should not complete an empty --- opener', () => {
+    const input = '---\n'
+    const expected = '---\n'
     expect(autoCloseMarkdown(input)).toBe(expected)
   })