fix(markdown-editor): preserve LaTeX backslashes for KaTeX rendering

analyzeMarkdownEditability canonicalizes markdown through a tiptap
round-trip that calls escapeMarkdownPlainText, which doubles every
backslash. This turns LaTeX commands like \int into \\int, \frac into
\\frac, etc. KaTeX interprets \\ as a line break, breaking math
rendering in the file viewer preview.

Fix: use raw markdown content instead of canonicalMarkdown in both
content normalization paths (toNormalizedMarkdown and initialContent
loading). The tiptap editor normalizes content internally, so the
canonical form is not needed for display.

Add a root-cause test in tiptapMarkdown.test.ts documenting that
canonicalMarkdown doubles backslashes in LaTeX math content.

Closes #1952

Author: xlx1212

src/web-ui/src/tools/editor/components/MarkdownEditor.tsx

@@ -143,8 +143,10 @@ const MarkdownEditor: React.FC<MarkdownEditorProps> = ({
 
   const toNormalizedMarkdown = useCallback((raw: string) => {
     const nextEditability = analyzeMarkdownEditability(raw);
-    const nextContent =
-      nextEditability.mode === 'unsafe' ? raw : nextEditability.canonicalMarkdown;
+    // Use raw content instead of canonicalMarkdown to avoid doubling
+    // backslashes in LaTeX commands (e.g. \int → \\int), which breaks
+    // KaTeX rendering. The tiptap editor normalizes content internally.
+    const nextContent = raw;
     return { nextEditability, nextContent };
   }, []);
 
@@ -262,9 +264,9 @@ const MarkdownEditor: React.FC<MarkdownEditorProps> = ({
       }
     } else if (initialContent !== undefined) {
       const nextEditability = analyzeMarkdownEditability(initialContent);
-      const nextContent = nextEditability.mode === 'unsafe'
-        ? initialContent
-        : nextEditability.canonicalMarkdown;
+      // Use raw content instead of canonicalMarkdown to avoid doubling
+      // backslashes in LaTeX commands, which breaks KaTeX rendering.
+      const nextContent = initialContent;
 
       setEditability(nextEditability);
       setContent(nextContent);

src/web-ui/src/tools/editor/meditor/utils/tiptapMarkdown.test.ts

@@ -334,4 +334,18 @@ describe('tiptap markdown compatibility', () => {
     expect(analysis.containsRawHtmlBlocks).toBe(true);
     expect(doc.content?.[0]?.type).toBe('rawHtmlBlock');
   });
+
+  it('doubles backslashes in LaTeX math during canonical round-trip', () => {
+    const markdown = String.raw`$$\int_0^1 x^2 dx = \frac13$$`;
+    const analysis = analyzeMarkdownEditability(markdown);
+
+    // Root cause of #1952: escapeMarkdownPlainText doubles every backslash
+    // during canonical serialization, turning \int into \\int, \frac into
+    // \\frac, etc. KaTeX interprets \\ as a line break, so the math renders
+    // incorrectly. This is why MarkdownEditor uses raw content instead of
+    // canonicalMarkdown for display.
+    expect(analysis.canonicalMarkdown).toContain(String.raw`\\int`);
+    expect(analysis.canonicalMarkdown).toContain(String.raw`\\frac`);
+    expect(analysis.canonicalMarkdown).not.toBe(markdown);
+  });
 });