[eric] chat: burst commits type out too (fresh whole-message commits route through the same smooth reveal)

frontend/src/app/pages/AgentChat/AgentChat.tsx

@@ -53,6 +53,7 @@ import WelcomeQuickReplies from './WelcomeQuickReplies';
 import { useWelcomeGreeting } from './useWelcomeGreeting';
 import { THINKING_LABELS } from './thinkingLabels';
 import MessageBubble from './bubbles/MessageBubble';
+import BurstRevealBubble from './bubbles/BurstRevealBubble';
 import { estimateRenderedTextHeight, RECHECK_VISIBILITY_EVENT } from './bubbles/markdownMeasure';
 import CompactionMarker from './bubbles/CompactionMarker';
 import MessageActionBar from './shell/MessageActionBar';
@@ -630,7 +631,12 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
     };
   }, [id, session?.id, scheduleWindowRecompute]);
 
+  // Burst-reveal bookkeeping: ids present at open (or after a session/branch hop) are HISTORY and
+  // never animate; only ids that appear later, while the agent is working, type themselves out.
+  const seenMessageIdsRef = useRef<Set<string> | null>(null);
+
   React.useLayoutEffect(() => {
+    seenMessageIdsRef.current = null;
     const seed = initialSeedItems(viewportHeight);
     const total = renderItemsLengthRef.current;
     const end = total;
@@ -1715,6 +1721,18 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
               }
               const msg = item;
               const isEditing = editingMessageId === msg.id;
+              // First pass after open/hop seeds the set; later unseen assistant ids arrived live.
+              if (seenMessageIdsRef.current === null) {
+                seenMessageIdsRef.current = new Set(renderItems.map((it) => it.id));
+              }
+              const burstAnimate =
+                msg.role === 'assistant' &&
+                typeof msg.content === 'string' &&
+                !isEditing &&
+                !seenMessageIdsRef.current.has(msg.id) &&
+                msg.id !== justStreamedId &&
+                (sessionRunning || awaitingResponse);
+              seenMessageIdsRef.current.add(msg.id);
               const siblings = getSiblingBranches(msg.id);
               const hasBranches = siblings.length > 0;
               const currentBranchIdx = hasBranches
@@ -1732,15 +1750,26 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
                     containIntrinsicSize: 'auto 120px',
                   }}
                 >
-                  <MessageBubble
-                    message={msg}
-                    editing={isEditing}
-                    onSaveEdit={handleSaveEdit}
-                    onCancelEdit={handleCancelEdit}
-                    viewportHeight={viewportHeight}
-                    viewportWidth={viewportWidth}
-                    scrollRoot={scrollRoot}
-                  />
+                  {msg.role === 'assistant' && typeof msg.content === 'string' && !isEditing ? (
+                    <BurstRevealBubble
+                      message={msg}
+                      animate={burstAnimate}
+                      onGrew={stickToBottomIfNeeded}
+                      viewportHeight={viewportHeight}
+                      viewportWidth={viewportWidth}
+                      scrollRoot={scrollRoot}
+                    />
+                  ) : (
+                    <MessageBubble
+                      message={msg}
+                      editing={isEditing}
+                      onSaveEdit={handleSaveEdit}
+                      onCancelEdit={handleCancelEdit}
+                      viewportHeight={viewportHeight}
+                      viewportWidth={viewportWidth}
+                      scrollRoot={scrollRoot}
+                    />
+                  )}
                   {!isEditing && (msg.role === 'user' || (msg.role === 'assistant' && lastAssistantIdsInTurn.has(msg.id))) && (
                     <MessageActionBar
                       role={msg.role as 'user' | 'assistant'}

frontend/src/app/pages/AgentChat/bubbles/BurstRevealBubble.tsx

@@ -0,0 +1,47 @@
+import React, { useEffect, useState } from 'react';
+import { AgentMessage } from '@/shared/state/agentsSlice';
+import MessageBubble from './MessageBubble';
+import { useSmoothText } from './useSmoothText';
+
+interface Props {
+  message: AgentMessage;
+  // Captured once at mount: a message that arrived whole mid-run types itself out; history never re-animates.
+  animate: boolean;
+  onGrew?: () => void;
+  viewportHeight?: number;
+  viewportWidth?: number;
+  scrollRoot?: Element | null;
+}
+
+/** Short post-tool answers often COMMIT whole and skip the streaming slice entirely, so they popped
+    while true streams typed. Route fresh commits through the same smooth reveal (assistant-ui's
+    drain pattern), then settle into the identical committed render so the handoff can't flash. */
+function BurstRevealBubble({ message, animate, onGrew, viewportHeight, viewportWidth, scrollRoot }: Props): React.ReactElement {
+  const [shouldAnimate] = useState(animate);
+  const full = typeof message.content === 'string' ? message.content : '';
+  const [done, setDone] = useState(!shouldAnimate || full.length === 0);
+  const { text, revealRef } = useSmoothText(full, !done);
+  useEffect(() => {
+    if (!done && text.length >= full.length) setDone(true);
+  }, [done, text.length, full.length]);
+  const grewRef = React.useRef(onGrew);
+  grewRef.current = onGrew;
+  useEffect(() => {
+    if (!done) grewRef.current?.();
+  }, [text.length, done]);
+  if (done) {
+    return <MessageBubble message={message} viewportHeight={viewportHeight} viewportWidth={viewportWidth} scrollRoot={scrollRoot} />;
+  }
+  return (
+    <MessageBubble
+      message={{ ...message, content: text }}
+      isStreaming
+      revealRef={revealRef}
+      viewportHeight={viewportHeight}
+      viewportWidth={viewportWidth}
+      scrollRoot={scrollRoot}
+    />
+  );
+}
+
+export default BurstRevealBubble;