🤖 Add up arrow to edit last user message (#272)

Pressing up arrow on an empty input now edits the last user message,
providing a quick way to iterate without clicking edit.

## Changes

- **ChatInput**: Added keyboard handler for up arrow when input is empty
- **AIView**: Added `handleEditLastUserMessage` callback that finds last
user message

## Cleanup

While implementing this, also cleaned up related code:
- Fixed placeholder to use `CANCEL_EDIT` keybind instead of `CANCEL`
- Removed redundant empty `CANCEL` keybind handler
- Removed redundant `NEW_LINE` handler (`matchesKeybind` already handles
Shift+Enter)

## Testing

Manually test:
1. Send a message
2. Clear input and press up arrow → should edit last message
3. Type something and press up arrow → should move cursor normally
4. Press Shift+Enter → should still insert newline

---
_Generated with `cmux`_

Author: ammar-agent

src/components/AIView.tsx

@@ -256,6 +256,26 @@ const AIViewInner: React.FC<AIViewProps> = ({
     setEditingMessage({ id: messageId, content });
   }, []);
 
+  const handleEditLastUserMessage = useCallback(() => {
+    if (!workspaceState) return;
+    const mergedMessages = mergeConsecutiveStreamErrors(workspaceState.messages);
+    const lastUserMessage = [...mergedMessages]
+      .reverse()
+      .find((msg): msg is Extract<DisplayedMessage, { type: "user" }> => msg.type === "user");
+    if (lastUserMessage) {
+      setEditingMessage({ id: lastUserMessage.historyId, content: lastUserMessage.content });
+      setAutoScroll(false); // Show jump-to-bottom indicator
+
+      // Scroll to the message being edited
+      requestAnimationFrame(() => {
+        const element = contentRef.current?.querySelector(
+          `[data-message-id="${lastUserMessage.historyId}"]`
+        );
+        element?.scrollIntoView({ behavior: "smooth", block: "center" });
+      });
+    }
+  }, [workspaceState, contentRef, setAutoScroll]);
+
   const handleCancelEdit = useCallback(() => {
     setEditingMessage(undefined);
   }, []);
@@ -464,12 +484,18 @@ const AIViewInner: React.FC<AIViewProps> = ({
 
                     return (
                       <React.Fragment key={msg.id}>
-                        <MessageRenderer
-                          message={msg}
-                          onEditUserMessage={handleEditUserMessage}
-                          workspaceId={workspaceId}
-                          isCompacting={isCompacting}
-                        />
+                        <div
+                          data-message-id={
+                            msg.type !== "history-hidden" ? msg.historyId : undefined
+                          }
+                        >
+                          <MessageRenderer
+                            message={msg}
+                            onEditUserMessage={handleEditUserMessage}
+                            workspaceId={workspaceId}
+                            isCompacting={isCompacting}
+                          />
+                        </div>
                         {isAtCutoff && (
                           <EditBarrier>
                             ⚠️ Messages below this line will be removed when you submit the edit
@@ -532,6 +558,7 @@ const AIViewInner: React.FC<AIViewProps> = ({
             isCompacting={isCompacting}
             editingMessage={editingMessage}
             onCancelEdit={handleCancelEdit}
+            onEditLastUserMessage={handleEditLastUserMessage}
             canInterrupt={canInterrupt}
             onReady={handleChatInputReady}
           />

src/components/ChatInput.tsx

@@ -129,6 +129,7 @@ export interface ChatInputProps {
   isCompacting?: boolean;
   editingMessage?: { id: string; content: string };
   onCancelEdit?: () => void;
+  onEditLastUserMessage?: () => void;
   canInterrupt?: boolean; // Whether Esc can be used to interrupt streaming
   onReady?: (api: ChatInputAPI) => void; // Callback with focus method
 }
@@ -335,6 +336,7 @@ export const ChatInput: React.FC<ChatInputProps> = ({
   isCompacting = false,
   editingMessage,
   onCancelEdit,
+  onEditLastUserMessage,
   canInterrupt = false,
   onReady,
 }) => {
@@ -809,13 +811,16 @@ export const ChatInput: React.FC<ChatInputProps> = ({
       }
     }
 
-    // Handle escape - let VimTextArea handle it (for Vim mode transitions)
-    // Edit canceling is handled by Ctrl+Q above
-    // Stream interruption is handled by Ctrl+C (INTERRUPT_STREAM keybind)
-    if (matchesKeybind(e, KEYBINDS.CANCEL)) {
-      // Do not preventDefault here: allow VimTextArea or other handlers (like suggestions) to process ESC
+    // Handle up arrow on empty input - edit last user message
+    if (e.key === "ArrowUp" && !editingMessage && input.trim() === "" && onEditLastUserMessage) {
+      e.preventDefault();
+      onEditLastUserMessage();
+      return;
     }
 
+    // Note: ESC handled by VimTextArea (for mode transitions) and CommandSuggestions (for dismissal)
+    // Edit canceling is Ctrl+Q, stream interruption is Ctrl+C
+
     // Don't handle keys if command suggestions are visible
     if (
       showCommandSuggestions &&
@@ -825,13 +830,7 @@ export const ChatInput: React.FC<ChatInputProps> = ({
       return; // Let CommandSuggestions handle it
     }
 
-    // Handle newline
-    if (matchesKeybind(e, KEYBINDS.NEW_LINE)) {
-      // Allow newline (default behavior)
-      return;
-    }
-
-    // Handle send message
+    // Handle send message (Shift+Enter for newline is default behavior)
     if (matchesKeybind(e, KEYBINDS.SEND_MESSAGE)) {
       e.preventDefault();
       void handleSend();
@@ -841,7 +840,7 @@ export const ChatInput: React.FC<ChatInputProps> = ({
   // Build placeholder text based on current state
   const placeholder = (() => {
     if (editingMessage) {
-      return `Edit your message... (${formatKeybind(KEYBINDS.CANCEL)} to cancel edit, ${formatKeybind(KEYBINDS.SEND_MESSAGE)} to send)`;
+      return `Edit your message... (${formatKeybind(KEYBINDS.CANCEL_EDIT)} to cancel, ${formatKeybind(KEYBINDS.SEND_MESSAGE)} to send)`;
     }
     if (isCompacting) {
       return `Compacting... (${formatKeybind(KEYBINDS.INTERRUPT_STREAM)} to cancel)`;