fix: preserve Markdown blocks when copying messages (#2944)

* fix: preserve Markdown blocks when copying messages

* fix: preserve blank lines when copying code

---------

Co-authored-by: Iuliia Ivashko <yulia.ivashko@gmail.com>

packages/ui/src/lib/clipboard.test.ts

@@ -1,6 +1,9 @@
 import { afterEach, describe, expect, test } from 'bun:test';
 
+import { marked } from 'marked';
+
 import { copyMarkdownToClipboard } from './clipboard';
+import { flattenAssistantTextParts } from './messages/messageText';
 
 const originalNavigator = Object.getOwnPropertyDescriptor(globalThis, 'navigator');
 const originalClipboardItem = Object.getOwnPropertyDescriptor(globalThis, 'ClipboardItem');
@@ -84,4 +87,52 @@ describe('copyMarkdownToClipboard', () => {
     expect(result).toEqual({ ok: true, method: 'clipboard' });
     expect(fallbackText).toBe('# title');
   });
+
+  test('assistant copy payload keeps Markdown block separation in every clipboard format', async () => {
+    let writtenItem: { data: Record<string, Blob> } | undefined;
+    class FakeClipboardItem {
+      static supports(type: string): boolean {
+        return type === 'text/markdown';
+      }
+
+      readonly data: Record<string, Blob>;
+
+      constructor(data: Record<string, Blob>) {
+        this.data = data;
+      }
+    }
+
+    Object.defineProperty(globalThis, 'ClipboardItem', { configurable: true, value: FakeClipboardItem });
+    Object.defineProperty(globalThis, 'navigator', {
+      configurable: true,
+      value: {
+        clipboard: {
+          write: async (items: Array<{ data: Record<string, Blob> }>) => {
+            writtenItem = items[0];
+          },
+        },
+      },
+    });
+
+    const parts = [
+      { id: 'p0', sessionID: 's', messageID: 'm', type: 'text', text: '第一段' },
+      { id: 'p1', sessionID: 's', messageID: 'm', type: 'text', text: '第二段' },
+      { id: 'p2', sessionID: 's', messageID: 'm', type: 'text', text: '```js\nconsole.log(1)\n\n\nconsole.log(2)\n```' },
+      { id: 'p3', sessionID: 's', messageID: 'm', type: 'text', text: '第三段' },
+    ];
+
+    // Same path as ChatMessage.tsx handleCopyMessage:
+    const text = flattenAssistantTextParts(parts as Parameters<typeof flattenAssistantTextParts>[0]);
+    const html = marked.parse(text, { gfm: true, breaks: false }) as string;
+    const result = await copyMarkdownToClipboard(text, html);
+
+    const expected = '第一段\n\n第二段\n\n```js\nconsole.log(1)\n\n\nconsole.log(2)\n```\n\n第三段';
+    expect(result).toEqual({ ok: true, method: 'clipboard' });
+    expect(await writtenItem?.data['text/plain']?.text()).toBe(expected);
+    expect(await writtenItem?.data['text/markdown']?.text()).toBe(expected);
+    const htmlText = await writtenItem?.data['text/html']?.text();
+    expect(htmlText).toContain('<p>第一段</p>');
+    expect(htmlText).toContain('<p>第二段</p>');
+    expect(htmlText).not.toContain('<p>第一段\n第二段</p>');
+  });
 });