fix: keep codexapp steer insert near streaming output

public/app.js

@@ -4652,7 +4652,9 @@
     const welcome = messagesDiv.querySelector('.welcome-msg');
     if (welcome) welcome.remove();
     messagesDiv.appendChild(createMsgElement('system', message, [], options));
-    scrollToBottom();
+    if (options.preserveScroll !== true) {
+      scrollToBottom();
+    }
   }
 
   function appendError(message, options = {}) {
@@ -4660,6 +4662,7 @@
       tone: 'danger',
       transient: options.transient !== false,
       autoDismissMs: options.autoDismissMs || 7000,
+      preserveScroll: options.preserveScroll !== false,
     });
   }
 
@@ -4670,6 +4673,17 @@
     });
   }
 
+  function isNearBottom(threshold = 96) {
+    const distance = messagesDiv.scrollHeight - messagesDiv.clientHeight - messagesDiv.scrollTop;
+    return distance <= threshold;
+  }
+
+  function scrollToBottomIfNear(threshold = 96) {
+    if (!isNearBottom(threshold)) return false;
+    scrollToBottom();
+    return true;
+  }
+
   // --- Custom Scrollbar ---
   const scrollbarEl = document.getElementById('custom-scrollbar');
   const thumbEl = document.getElementById('custom-scrollbar-thumb');
@@ -5334,10 +5348,20 @@
       }
       const messageId = createLocalId('user');
       const element = createMsgElement('user', text, [], { messageId, codexAppSteerStatus: 'pending' });
-      messagesDiv.appendChild(element);
+      const streamEl = document.getElementById('streaming-msg');
+      const shouldFollow = isNearBottom();
+      if (streamEl && streamEl.parentNode === messagesDiv) {
+        messagesDiv.insertBefore(element, streamEl);
+      } else {
+        messagesDiv.appendChild(element);
+      }
       registerUserMessage(messageId, element, text);
       updateUserOutlinePanel();
-      scrollToBottom();
+      if (shouldFollow) {
+        scrollToBottom();
+      } else {
+        updateScrollbar();
+      }
       send({ type: 'message', text, sessionId: currentSessionId, mode: currentMode, agent: currentAgent, clientMessageId: messageId });
       msgInput.value = '';
       autoResize();