Merge #4046 into stage-mj

static/boot.js

@@ -1288,8 +1288,8 @@ $('modelSelect').onchange=async()=>{
   }
 };
 $('msg').addEventListener('input',()=>{
-  autoResize();
   updateSendBtn();
+  scheduleComposerAutoResize();
   // Persist composer draft to server (debounced in _saveComposerDraft).
   const sid = S && S.session && S.session.session_id;
   if (sid && typeof _saveComposerDraft === 'function') {

static/messages.js

@@ -3868,7 +3868,25 @@ function transcript(){
   return lines.join('\n');
 }
 
-function autoResize(){const el=$('msg');el.style.height='auto';el.style.height=Math.min(el.scrollHeight,200)+'px';updateSendBtn();}
+let _composerAutoResizeRaf=0;
+function autoResize(){
+  if(_composerAutoResizeRaf && typeof cancelAnimationFrame==='function'){
+    cancelAnimationFrame(_composerAutoResizeRaf);
+    _composerAutoResizeRaf=0;
+  }
+  const el=$('msg');
+  el.style.height='auto';
+  el.style.height=Math.min(el.scrollHeight,200)+'px';
+  updateSendBtn();
+}
+function scheduleComposerAutoResize(){
+  if(typeof requestAnimationFrame!=='function'){autoResize();return;}
+  if(_composerAutoResizeRaf) return;
+  _composerAutoResizeRaf=requestAnimationFrame(()=>{
+    _composerAutoResizeRaf=0;
+    autoResize();
+  });
+}
 
 
 // ── YOLO mode state ──

tests/test_issue4042_typing_lag_autoresize.py

@@ -0,0 +1,27 @@
+from pathlib import Path
+
+
+REPO = Path(__file__).resolve().parents[1]
+MESSAGES_JS = (REPO / "static" / "messages.js").read_text(encoding="utf-8")
+BOOT_JS = (REPO / "static" / "boot.js").read_text(encoding="utf-8")
+
+
+def test_composer_autoresize_is_single_flight_per_frame():
+    assert "let _composerAutoResizeRaf=0;" in MESSAGES_JS
+    assert "function scheduleComposerAutoResize()" in MESSAGES_JS
+    assert "if(typeof requestAnimationFrame!=='function'){autoResize();return;}" in MESSAGES_JS
+    assert "if(_composerAutoResizeRaf) return;" in MESSAGES_JS
+    assert "_composerAutoResizeRaf=requestAnimationFrame(()=>{" in MESSAGES_JS
+    assert "cancelAnimationFrame(_composerAutoResizeRaf);" in MESSAGES_JS
+    assert "_composerAutoResizeRaf=0;" in MESSAGES_JS
+    assert "autoResize();" in MESSAGES_JS
+
+
+def test_composer_input_listener_uses_scheduler_instead_of_direct_reflow():
+    start = BOOT_JS.index("$('msg').addEventListener('input',()=>{")
+    end = BOOT_JS.index("// Persist composer draft to server", start)
+    listener = BOOT_JS[start:end]
+
+    assert "scheduleComposerAutoResize();" in listener
+    assert "autoResize();" not in listener
+    assert "updateSendBtn();" in listener