fix(editor): hide YAML frontmatter in Markdown preview

src/client/markdown-frontmatter.ts

@@ -0,0 +1,29 @@
+/**
+ * Return the Markdown source used by preview renderers.
+ *
+ * A closed YAML frontmatter block is metadata, but the shared MarkdownText
+ * parser treats its delimiters as a thematic break and a setext heading. Hide
+ * only a block that starts on the first line and has its own closing `---`
+ * line. Unclosed or non-leading delimiters remain byte-for-byte unchanged so
+ * ordinary Markdown is never truncated. The editor and save path keep using
+ * the original source; this helper is preview-only.
+ */
+export function markdownPreviewSource(source: string): string {
+  const firstLineEnd = source.indexOf('\n')
+  if (firstLineEnd === -1) return source
+
+  const firstLineStart = source.charCodeAt(0) === 0xFEFF ? 1 : 0
+  const firstLine = source.slice(firstLineStart, firstLineEnd).replace(/\r$/u, '')
+  if (firstLine !== '---') return source
+
+  let lineStart = firstLineEnd + 1
+  while (lineStart <= source.length) {
+    const newline = source.indexOf('\n', lineStart)
+    const lineEnd = newline === -1 ? source.length : newline
+    const line = source.slice(lineStart, lineEnd).replace(/\r$/u, '')
+    if (line === '---') return newline === -1 ? '' : source.slice(newline + 1)
+    if (newline === -1) return source
+    lineStart = newline + 1
+  }
+  return source
+}