Fix markdown parsing for code blocks when withCodeBlocks is false

front_end/src/components/markdown_editor/__tests__/initialized_editor.test.tsx

@@ -172,6 +172,23 @@ describe("InitializedMarkdownEditor", () => {
     });
   });
 
+  describe("code blocks", () => {
+    it("renders existing code blocks in write mode when withCodeBlocks is false", async () => {
+      // Given
+      const markdown = "Before\n\n```\nconsole.log('hi');\n```\n\nAfter";
+
+      // When / Then — must not throw "Parsing of the following markdown
+      // structure failed: {type: code, name: N/A}".
+      await renderWithAct(
+        <MockedEditorComponent markdown={markdown} mode="write" />
+      );
+
+      const editor = screen.getByLabelText(EDITOR_LABEL);
+      expect(editor).toHaveTextContent("Before");
+      expect(editor).toHaveTextContent("After");
+    });
+  });
+
   describe("html tags", () => {
     it("properly preserves HTML tags with custom attributes", async () => {
       // Given

front_end/src/components/markdown_editor/initialized_editor.tsx

@@ -258,11 +258,11 @@ const InitializedMarkdownEditor: FC<
       equationPlugin(),
     ];
 
-    if (!withCodeBlocks) {
-      return common;
-    }
-
-    if (mode === "read") {
+    // Always register codeBlockPlugin so pre-existing ``` fences in the source
+    // markdown parse — otherwise MDXEditor throws on unknown "code" nodes.
+    // `withCodeBlocks` only gates the insert-code-block toolbar affordance and
+    // the rich CodeMirror editing experience.
+    if (!withCodeBlocks || mode === "read") {
       return [
         ...common,
         codeBlockPlugin({