From 1498335e398f2cd23b8fd1133ca3ba020d9b8905 Mon Sep 17 00:00:00 2001
From: "cursor[bot]" <206951365+cursor[bot]@users.noreply.github.com>
Date: Tue, 5 May 2026 20:57:33 -0700
Subject: [PATCH] Optimize MessagesTimeline work row stability (#2527)

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Julius Marminge <juliusmarminge@users.noreply.github.com>
---
 .../chat/MessagesTimeline.logic.test.ts       | 54 +++++++++++++++++++
 .../components/chat/MessagesTimeline.logic.ts |  3 +-
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/apps/web/src/components/chat/MessagesTimeline.logic.test.ts b/apps/web/src/components/chat/MessagesTimeline.logic.test.ts
index 7dc6583b133b1aeaa67cc27c3aed41947156e168..633fb5d6bef0b73efd0735c4a0899084deb144cb 100644
--- a/apps/web/src/components/chat/MessagesTimeline.logic.test.ts
+++ b/apps/web/src/components/chat/MessagesTimeline.logic.test.ts
@@ -382,6 +382,60 @@ describe("computeStableMessagesTimelineRows", () => {
     expect(repeated.result).toBe(initial.result);
   });
 
+  it("reuses work rows when equivalent timeline derivations create new grouped arrays", () => {
+    const firstWorkEntry = {
+      id: "work-1",
+      createdAt: "2026-01-01T00:00:00Z",
+      label: "thinking",
+      detail: "Inspecting repository state",
+      tone: "thinking" as const,
+    };
+    const secondWorkEntry = {
+      id: "work-2",
+      createdAt: "2026-01-01T00:00:01Z",
+      label: "read",
+      detail: "Reading package.json",
+      tone: "tool" as const,
+    };
+
+    const createRows = () =>
+      deriveMessagesTimelineRows({
+        timelineEntries: [
+          {
+            id: "entry-work-1",
+            kind: "work",
+            createdAt: firstWorkEntry.createdAt,
+            entry: firstWorkEntry,
+          },
+          {
+            id: "entry-work-2",
+            kind: "work",
+            createdAt: secondWorkEntry.createdAt,
+            entry: secondWorkEntry,
+          },
+        ],
+        completionDividerBeforeEntryId: null,
+        isWorking: false,
+        activeTurnStartedAt: null,
+        turnDiffSummaryByAssistantMessageId: new Map(),
+        revertTurnCountByUserMessageId: new Map(),
+      });
+
+    const firstRows = createRows();
+    const initial = computeStableMessagesTimelineRows(firstRows, {
+      byId: new Map(),
+      result: [],
+    });
+    const secondRows = createRows();
+
+    expect(secondRows[0]).not.toBe(firstRows[0]);
+
+    const repeated = computeStableMessagesTimelineRows(secondRows, initial);
+
+    expect(repeated).toBe(initial);
+    expect(repeated.result[0]).toBe(initial.result[0]);
+  });
+
   it("returns a new result when row order changes without content changes", () => {
     const firstUserMessage = {
       id: "user-1" as never,
diff --git a/apps/web/src/components/chat/MessagesTimeline.logic.ts b/apps/web/src/components/chat/MessagesTimeline.logic.ts
index a56643d84b0d3c074e286350eb4868046ef86061..c99f0b98a66e827469e43917a002dd1031223901 100644
--- a/apps/web/src/components/chat/MessagesTimeline.logic.ts
+++ b/apps/web/src/components/chat/MessagesTimeline.logic.ts
@@ -1,3 +1,4 @@
+import * as Equal from "effect/Equal";
 import { type TimelineEntry, type WorkLogEntry } from "../../session-logic";
 import { type ChatMessage, type ProposedPlan, type TurnDiffSummary } from "../../types";
 import { type MessageId } from "@t3tools/contracts";
@@ -223,7 +224,7 @@ function isRowUnchanged(a: MessagesTimelineRow, b: MessagesTimelineRow): boolean
       return a.proposedPlan === (b as typeof a).proposedPlan;
 
     case "work":
-      return a.groupedEntries === (b as typeof a).groupedEntries;
+      return Equal.equals(a.groupedEntries, (b as typeof a).groupedEntries);
 
     case "message": {
       const bm = b as typeof a;
-- 
2.54.0