test(client): pin first-line-only frontmatter detection

packages/client/ui-sidebar-documentpreview/src/client/markdown/frontmatter.ts

@@ -6,8 +6,8 @@ export interface FrontmatterSplit {
   body: string
 }
 
-// Opening `---` on the first line, then the first closing `---` or `...` line; `??` lets an
-// empty block close on the line right after the opening.
+// Only an opening `---` on the first line (after an optional BOM) counts; the block ends at the
+// first closing `---` or `...` line, and `??` lets an empty block close on the next line.
 const FRONTMATTER = /^\uFEFF?---[ \t]*\r?\n(?:([\s\S]*?)\r?\n)??(?:---|\.\.\.)[ \t]*(?:\r?\n|$)/u
 
 /**

packages/client/ui-sidebar-documentpreview/tests/markdown-body.client.spec.tsx

@@ -173,6 +173,16 @@ describe('MarkdownBody', () => {
     expect(view.container.querySelector('hr')).not.toBeNull()
   })
 
+  it.each([
+    ['a leading blank line', '\n---\nname: pdf\n---\nBody.'],
+    ['leading text', 'Intro.\n\n---\nname: pdf\n---\nBody.'],
+    ['an indented opening rule', ' ---\nname: pdf\n---\nBody.'],
+  ])('treats a delimiter pair after %s as Markdown', (_, text) => {
+    const view = render(<MarkdownBody {...props(content([text], true))} />)
+    expect(view.container.querySelector('[data-document-frontmatter]')).toBeNull()
+    expect(view.getByText('Body.')).toBeDefined()
+  })
+
   it('renders empty text and leaves non-text deliveries to their selected implementation', () => {
     const view = render(<MarkdownBody {...props({ kind: 'text', text: '', pages: [], eof: true })} />)
     expect(view.container.querySelector('[data-document-markdown]')?.textContent).toBe('')