From 87dbc59bf18016d094c5ad9861c67044831444b3 Mon Sep 17 00:00:00 2001
From: Serhii Dziupin <serkraser@gmail.com>
Date: Fri, 7 Aug 2026 09:16:34 +0300
Subject: [PATCH] fix(chat): do not replay entry animations for already-seen
 fresh messages (#2124) (#2732)

---
 packages/ui/src/lib/messageFreshness.test.ts | 85 ++++++++++++++++++++
 packages/ui/src/lib/messageFreshness.ts      | 11 ++-
 2 files changed, 92 insertions(+), 4 deletions(-)
 create mode 100644 packages/ui/src/lib/messageFreshness.test.ts

diff --git a/packages/ui/src/lib/messageFreshness.test.ts b/packages/ui/src/lib/messageFreshness.test.ts
new file mode 100644
index 00000000..bd59bffd
--- /dev/null
+++ b/packages/ui/src/lib/messageFreshness.test.ts
@@ -0,0 +1,85 @@
+import { beforeEach, describe, expect, test } from 'bun:test';
+
+import { MessageFreshnessDetector } from './messageFreshness';
+
+import type { Message } from '@opencode-ai/sdk/v2';
+
+const makeAssistantMessage = (id: string, created: number): Message =>
+    ({
+        id,
+        role: 'assistant',
+        sessionID: 'session-a',
+        time: { created },
+    }) as unknown as Message;
+
+describe('MessageFreshnessDetector.shouldAnimateMessage', () => {
+    let detector: MessageFreshnessDetector;
+
+    beforeEach(() => {
+        detector = MessageFreshnessDetector.getInstance();
+        detector.clearAll();
+    });
+
+    test('fresh message animates once and is recorded as seen', () => {
+        detector.recordSessionStart('session-a');
+        const message = makeAssistantMessage('msg-fresh', Date.now());
+
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(true);
+        expect(detector.hasBeenAnimated('msg-fresh')).toBe(true);
+    });
+
+    test('regression #2124: fresh message does not re-animate when returning to the session', () => {
+        detector.recordSessionStart('session-a');
+        const message = makeAssistantMessage('msg-fresh', Date.now());
+
+        // First visit: the message is fresh and animates.
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(true);
+
+        // User switches away and back; ChatViewport remounts and re-evaluates
+        // before recordSessionStart runs again, so the old session start time
+        // is still in effect. The message must not animate a second time.
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(false);
+    });
+
+    test('stale history message never animates and is recorded as seen', () => {
+        detector.recordSessionStart('session-a');
+        const message = makeAssistantMessage('msg-old', Date.now() - 60_000);
+
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(false);
+        expect(detector.hasBeenAnimated('msg-old')).toBe(true);
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(false);
+    });
+
+    test('message evaluated without a recorded session start does not animate and is recorded', () => {
+        const message = makeAssistantMessage('msg-no-session', Date.now());
+
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(false);
+        expect(detector.hasBeenAnimated('msg-no-session')).toBe(true);
+
+        // Recording the session start afterwards must not resurrect the animation.
+        detector.recordSessionStart('session-a');
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(false);
+    });
+
+    test('non-assistant messages never animate', () => {
+        detector.recordSessionStart('session-a');
+        const message = {
+            id: 'msg-user',
+            role: 'user',
+            sessionID: 'session-a',
+            time: { created: Date.now() },
+        } as unknown as Message;
+
+        expect(detector.shouldAnimateMessage(message, 'session-a')).toBe(false);
+    });
+
+    test('a new fresh message still animates after older fresh messages were seen', () => {
+        detector.recordSessionStart('session-a');
+        const first = makeAssistantMessage('msg-first', Date.now());
+        const second = makeAssistantMessage('msg-second', Date.now());
+
+        expect(detector.shouldAnimateMessage(first, 'session-a')).toBe(true);
+        expect(detector.shouldAnimateMessage(second, 'session-a')).toBe(true);
+        expect(detector.shouldAnimateMessage(second, 'session-a')).toBe(false);
+    });
+});
diff --git a/packages/ui/src/lib/messageFreshness.ts b/packages/ui/src/lib/messageFreshness.ts
index e32c5843..230a747c 100644
--- a/packages/ui/src/lib/messageFreshness.ts
+++ b/packages/ui/src/lib/messageFreshness.ts
@@ -44,10 +44,13 @@ export class MessageFreshnessDetector {
 
         const isFresh = message.time.created > (sessionStartTime - 5000);
 
-        if (!isFresh) {
-            this.seenMessageIds.add(message.id);
-            this.messageCreationTimes.set(message.id, message.time.created);
-        }
+        // Record fresh messages too so they animate at most once per detector
+        // lifetime. The detector is a module singleton that outlives ChatViewport
+        // remounts; without this, switching away and back re-evaluates the same
+        // message against the stale session start time (recordSessionStart runs
+        // in an effect after the first render) and replays the entry animation.
+        this.seenMessageIds.add(message.id);
+        this.messageCreationTimes.set(message.id, message.time.created);
 
         return isFresh;
     }