feat(chat): recall last user message on empty composer ArrowUp (#1175)

Pressing ArrowUp on an empty #message composer restores the last sent user text, matching common chat-app UX (Slack, Discord, ChatGPT).

- Read from #chat-history .msg-user dataset.raw (same path as resend/regenerate), not session sidebar metadata

- Literal empty check (whitespace-only drafts are preserved); ignore Shift/Alt/Ctrl/Meta and IME composition

- Extract wiring to composerArrowUpRecall.js; rAF + 250ms retry only (no global MutationObserver)

- Add tests/test_composer_arrow_up_recall_js.py

Co-authored-by: Cursor <cursoragent@cursor.com>

static/js/composerArrowUpRecall.js

@@ -0,0 +1,61 @@
+/**
+ * ArrowUp on an empty composer recalls the last user message (chat-app convention).
+ */
+
+/**
+ * Last user bubble in the active chat surface (#chat-history), using dataset.raw
+ * (same source as resend/regenerate in chat.js).
+ *
+ * @param {Document | Element} [root=document]
+ * @returns {string}
+ */
+export function getLastUserMessageFromChatHistory(root = document) {
+  const chatBox =
+    root && root.id === 'chat-history' && typeof root.querySelectorAll === 'function'
+      ? root
+      : (root.getElementById ? root.getElementById('chat-history') : null);
+  if (!chatBox) return '';
+
+  const users = chatBox.querySelectorAll('.msg-user');
+  const last = users[users.length - 1];
+  if (!last) return '';
+
+  const bodyEl = last.querySelector('.body');
+  return last.dataset?.raw || (bodyEl ? bodyEl.textContent : '') || '';
+}
+
+/**
+ * @param {HTMLTextAreaElement} composer
+ * @param {() => string} getLastUserMessage
+ * @param {{ autoResize?: (el: HTMLTextAreaElement) => void }} [options]
+ * @returns {boolean} true when wired (or already wired)
+ */
+export function wireArrowUpRecall(composer, getLastUserMessage, options = {}) {
+  if (!composer) return false;
+  if (composer._arrowUpRecallWired) return true;
+  composer._arrowUpRecallWired = true;
+
+  const { autoResize } = options;
+
+  composer.addEventListener('keydown', (e) => {
+    // Only ArrowUp, no modifier keys, no IME composition
+    if (e.key !== 'ArrowUp') return;
+    if (e.shiftKey || e.altKey || e.ctrlKey || e.metaKey) return;
+    if (e.isComposing) return;
+
+    // Literal emptiness — intentional whitespace is not empty
+    if (composer.value !== '') return;
+
+    const recalled = getLastUserMessage();
+    if (!recalled) return;
+
+    e.preventDefault();
+    composer.value = recalled;
+    try {
+      composer.selectionStart = composer.selectionEnd = recalled.length;
+    } catch (_) {}
+    if (autoResize) autoResize(composer);
+  });
+
+  return true;
+}