fix: handle Markdown paste only once

doc/plans/2026-10-02-pi-production-readiness.md

@@ -15,6 +15,17 @@ failures remain retained. The Mac file-edit correction also passes.
 
 ### Latest boundary fixes — 2026-10-05
 
+The explicit version-12 Mac memory attempt at `867fa4711` still times out
+at the unchanged native-session limit; cleanup passes. Its valid fenced JSON
+arrives in a duplicated description: the Markdown paste capture inserts the
+parsed content, then Lexical inserts the same plain text. The editor now stops
+that handled paste before it reaches the inner editor. A regression fails on
+the old propagation, and all 51 editor tests pass after the fix, including an
+ordinary-text paste control. The token gates pass. This is a new shipping input;
+the previous image and live result remain bound to `867fa4711`, and neither
+is qualification of the corrected editor. No paid retry is justified until a
+zero-model browser check verifies one submitted prompt with intact JSON bytes.
+
 Normal public installs of runtime `b012b3aebe` admit profile 15 on Mac ARM,
 Mac Intel (through Rosetta), and native Linux. The native Linux image is
 `sha256:3279d92405a59b4654cb6af5de27bfacffee73a2c57311bf5f2d5ff9272b9b67`.

ui/src/components/MarkdownEditor.test.tsx

@@ -912,6 +912,35 @@ describe("MarkdownEditor", () => {
     });
   });
 
+  it("handles Markdown paste once before the inner editor receives it", async () => {
+    const root = createRoot(container);
+    await act(async () => {
+      root.render(<MarkdownEditor value="" onChange={() => {}} />);
+    });
+    await flush();
+    const editable = container.querySelector('[data-testid="mdx-editor"]')!;
+    const innerPaste = vi.fn();
+    editable.addEventListener("paste", innerPaste);
+    const pasted = '```json\n{"content":"nonce\\n"}\n```';
+    const event = new Event("paste", { bubbles: true, cancelable: true });
+    Object.defineProperty(event, "clipboardData", {
+      value: { types: ["text/plain"], getData: () => pasted },
+    });
+    await act(async () => { editable.dispatchEvent(event); });
+    expect(event.defaultPrevented).toBe(true);
+    expect(mdxEditorMockState.insertedMarkdownValues).toEqual([pasted]);
+    expect(innerPaste).not.toHaveBeenCalled();
+
+    const plain = new Event("paste", { bubbles: true, cancelable: true });
+    Object.defineProperty(plain, "clipboardData", {
+      value: { types: ["text/plain"], getData: () => "ordinary text" },
+    });
+    await act(async () => { editable.dispatchEvent(plain); });
+    expect(plain.defaultPrevented).toBe(false);
+    expect(innerPaste).toHaveBeenCalledOnce();
+    await act(async () => { root.unmount(); });
+  });
+
   it("escapes angle brackets in pasted markdown", async () => {
     const root = createRoot(container);
 

ui/src/components/MarkdownEditor.tsx

@@ -1281,6 +1281,9 @@ export const MarkdownEditor = forwardRef<MarkdownEditorRef, MarkdownEditorProps>
     if (!looksLikeMarkdownPaste(rawText)) return;
 
     event.preventDefault();
+    // Lexical also handles paste on the editable element. Once Markdown is
+    // inserted here, prevent that handler from inserting the plain text again.
+    event.stopPropagation();
     ref.current.insertMarkdown(escapeUnsupportedAngleBrackets(normalizeMarkdown(rawText)));
   }, []);