diff --git a/apps/web/src/components/DiffPanel.tsx b/apps/web/src/components/DiffPanel.tsx
index a2678e03fcd39cd083a724f01645e5a492131f5e..43b19188671f31b4914797125cd4a413c9403a82 100644
--- a/apps/web/src/components/DiffPanel.tsx
+++ b/apps/web/src/components/DiffPanel.tsx
@@ -107,7 +107,7 @@ export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
   const patchViewportRef = useRef<HTMLDivElement>(null);
   const params = useParams({ strict: false });
   const routeThreadId = typeof params.threadId === "string" ? params.threadId : null;
-  const activeThreadId = state.diffThreadId ?? routeThreadId;
+  const activeThreadId = routeThreadId ?? state.diffThreadId;
   const activeThread = state.threads.find((thread) => thread.id === activeThreadId);
   const activeThreadRuntimeId =
     activeThread?.codexThreadId ?? activeThread?.session?.threadId ?? null;
@@ -216,6 +216,17 @@ export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
     target?.scrollIntoView({ block: "nearest" });
   }, [selectedFilePath, renderableFiles]);
 
+  useEffect(() => {
+    if (!state.diffOpen) return;
+    if (!routeThreadId) return;
+    if (state.diffThreadId === routeThreadId) return;
+
+    dispatch({
+      type: "SET_DIFF_TARGET",
+      threadId: routeThreadId,
+    });
+  }, [dispatch, routeThreadId, state.diffOpen, state.diffThreadId]);
+
   const selectTurn = (turnId: string) => {
     if (!activeThread) return;
     dispatch({
diff --git a/apps/web/src/store.test.ts b/apps/web/src/store.test.ts
index 5b3c7a81117c465e86caa5a4ad7579b87c0886aa..e0fa075b3097aabc70f0082dae68473dbc8a3c9f 100644
--- a/apps/web/src/store.test.ts
+++ b/apps/web/src/store.test.ts
@@ -833,6 +833,48 @@ describe("store reducer thread continuity", () => {
     expect(next.threads[0]?.lastVisitedAt).toBe(nextVisitedAt);
   });
 
+  it("retargets open diff state to the visited thread", () => {
+    const state: AppState = {
+      ...makeState(
+        makeThread({
+          id: "thread-local-1",
+          latestTurnCompletedAt: "2026-02-08T10:00:10.000Z",
+          lastVisitedAt: "2026-02-08T10:00:00.000Z",
+        }),
+      ),
+      threads: [
+        makeThread({
+          id: "thread-local-1",
+          latestTurnCompletedAt: "2026-02-08T10:00:10.000Z",
+          lastVisitedAt: "2026-02-08T10:00:00.000Z",
+        }),
+        makeThread({
+          id: "thread-local-2",
+          session: makeSession({ sessionId: "sess-2" }),
+          latestTurnCompletedAt: "2026-02-08T09:00:10.000Z",
+          lastVisitedAt: "2026-02-08T09:00:00.000Z",
+        }),
+      ],
+      diffOpen: true,
+      diffThreadId: "thread-local-1",
+      diffTurnId: "turn-1",
+      diffFilePath: "src/old.ts",
+    };
+
+    const nextVisitedAt = "2026-02-08T10:00:20.000Z";
+    const next = reducer(state, {
+      type: "MARK_THREAD_VISITED",
+      threadId: "thread-local-2",
+      visitedAt: nextVisitedAt,
+    });
+
+    expect(next.threads[1]?.lastVisitedAt).toBe(nextVisitedAt);
+    expect(next.diffOpen).toBe(true);
+    expect(next.diffThreadId).toBe("thread-local-2");
+    expect(next.diffTurnId).toBeNull();
+    expect(next.diffFilePath).toBeNull();
+  });
+
   it("reverts thread state to a checkpoint snapshot", () => {
     const state = makeState(
       makeThread({
diff --git a/apps/web/src/store.ts b/apps/web/src/store.ts
index f470a8c83bd5dba8c8eaa3a850e3e6e7c8bc60ab..2784547fe2436413b0db8403f34ec1464491b563 100644
--- a/apps/web/src/store.ts
+++ b/apps/web/src/store.ts
@@ -1078,12 +1078,16 @@ export function reducer(state: AppState, action: Action): AppState {
 
     case "MARK_THREAD_VISITED": {
       const visitedAt = action.visitedAt ?? new Date().toISOString();
+      const shouldRetargetOpenDiff = state.diffOpen && state.diffThreadId !== action.threadId;
       return {
         ...state,
         threads: updateThread(state.threads, action.threadId, (t) => ({
           ...t,
           lastVisitedAt: visitedAt,
         })),
+        diffThreadId: shouldRetargetOpenDiff ? action.threadId : state.diffThreadId,
+        diffTurnId: shouldRetargetOpenDiff ? null : state.diffTurnId,
+        diffFilePath: shouldRetargetOpenDiff ? null : state.diffFilePath,
       };
     }