fix(ui): preserve composer paste undo history

apps/desktop/e2e/composer-undo.spec.ts

@@ -0,0 +1,61 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import type { Locator } from '@playwright/test';
+import { expect, test, COMPOSER_INPUT } from './fixtures';
+
+const TYPED = 'x';
+const PASTED = '中文 <tag>& "quoted"\r\nhttps://example.test/path?x=1&y=2\n第二行 <>&';
+const PASTED_AS_PLAIN_TEXT = PASTED.replace(/\r\n/g, '\n');
+const UNDO_SHORTCUT = process.platform === 'darwin' ? 'Meta+z' : 'Control+z';
+
+async function expectComposerText(
+  composer: Locator,
+  expected: string,
+): Promise<void> {
+  await expect
+    .poll(() => composer.evaluate((element) => element.innerText.replace(/\r\n/g, '\n')))
+    .toBe(expected);
+}
+
+test('plain-text paste is undone separately from prior typing', async ({ window: page }) => {
+  const composer = page.locator(COMPOSER_INPUT);
+
+  await composer.click();
+  await page.keyboard.type(TYPED);
+  await expectComposerText(composer, TYPED);
+
+  await composer.evaluate((element, pasted) => {
+    const transfer = new DataTransfer();
+    transfer.setData('text/plain', pasted);
+    element.dispatchEvent(
+      new ClipboardEvent('paste', { bubbles: true, cancelable: true, clipboardData: transfer }),
+    );
+  }, PASTED);
+  await expectComposerText(composer, `${TYPED}${PASTED_AS_PLAIN_TEXT}`);
+
+  await page.keyboard.press(UNDO_SHORTCUT);
+  await expectComposerText(composer, TYPED);
+
+  await page.keyboard.press(UNDO_SHORTCUT);
+  await expectComposerText(composer, '');
+
+  await page.keyboard.press(UNDO_SHORTCUT);
+  await expectComposerText(composer, '');
+});

packages/ui/src/composer.tsx

@@ -1711,6 +1711,20 @@ export const Composer = forwardRef<
                 hasHistory={false}
                 triggers={triggers}
                 pasteAsToken={pasteAsToken}
+                onPaste={(_event, pasted) => {
+                  // Astryx has already offered token-adjacent, file, and
+                  // reference-sized-token pastes before it reaches this seam.
+                  // `insertHTML` creates a browser undo transaction distinct
+                  // from the keyboard input immediately before the paste.
+                  if (props.disabled || isReferenceSizedPaste(pasted)) return false;
+                  const plainTextContainer = document.createElement('div');
+                  plainTextContainer.textContent = pasted;
+                  return document.execCommand(
+                    'insertHTML',
+                    false,
+                    plainTextContainer.innerHTML.replace(/\r\n?|\n/g, '<br>'),
+                  );
+                }}
                 onFiles={onInputFiles}
                 onKeyDown={onInputKeyDown}
                 onCompositionStart={() => { compositionActiveRef.current = true; }}