diff --git a/frontend/src/app/activity/page.tsx b/frontend/src/app/activity/page.tsx
index 2b2c30a..73257eb 100644
--- a/frontend/src/app/activity/page.tsx
+++ b/frontend/src/app/activity/page.tsx
@@ -20,6 +20,7 @@ import { Button } from "@/components/ui/button";
 import { createExponentialBackoff } from "@/lib/backoff";
 import { apiDatetimeToMs, parseApiDatetime } from "@/lib/datetime";
 import { cn } from "@/lib/utils";
+import { usePageActive } from "@/hooks/usePageActive";
 
 const SSE_RECONNECT_BACKOFF = {
   baseMs: 1_000,
@@ -132,6 +133,7 @@ FeedCard.displayName = "FeedCard";
 
 export default function ActivityPage() {
   const { isSignedIn } = useAuth();
+  const isPageActive = usePageActive();
 
   const feedQuery = useListTaskCommentFeedApiV1ActivityTaskCommentsGet<
     listTaskCommentFeedApiV1ActivityTaskCommentsGetResponse,
@@ -189,6 +191,7 @@ export default function ActivityPage() {
   }, []);
 
   useEffect(() => {
+    if (!isPageActive) return;
     if (!isSignedIn) return;
     let isCancelled = false;
     const abortController = new AbortController();
@@ -278,7 +281,7 @@ export default function ActivityPage() {
         window.clearTimeout(reconnectTimeout);
       }
     };
-  }, [isSignedIn, pushFeedItem]);
+  }, [isPageActive, isSignedIn, pushFeedItem]);
 
   const orderedFeed = useMemo(() => {
     return [...feedItems].sort((a, b) => {
diff --git a/frontend/src/app/boards/[boardId]/page.tsx b/frontend/src/app/boards/[boardId]/page.tsx
index 1725560..b0e5f1f 100644
--- a/frontend/src/app/boards/[boardId]/page.tsx
+++ b/frontend/src/app/boards/[boardId]/page.tsx
@@ -72,6 +72,7 @@ import type {
 import { createExponentialBackoff } from "@/lib/backoff";
 import { apiDatetimeToMs, parseApiDatetime } from "@/lib/datetime";
 import { cn } from "@/lib/utils";
+import { usePageActive } from "@/hooks/usePageActive";
 
 type Board = BoardRead;
 
@@ -298,6 +299,7 @@ export default function BoardDetailPage() {
   const boardIdParam = params?.boardId;
   const boardId = Array.isArray(boardIdParam) ? boardIdParam[0] : boardIdParam;
   const { isSignedIn } = useAuth();
+  const isPageActive = usePageActive();
   const taskIdFromUrl = searchParams.get("taskId");
 
   const [board, setBoard] = useState<Board | null>(null);
@@ -580,7 +582,9 @@ export default function BoardDetailPage() {
   };
 
   useEffect(() => {
+    if (!isPageActive) return;
     if (!isSignedIn || !boardId || !board) return;
+    if (!isChatOpen) return;
     let isCancelled = false;
     const abortController = new AbortController();
     const backoff = createExponentialBackoff(SSE_RECONNECT_BACKOFF);
@@ -685,9 +689,10 @@ export default function BoardDetailPage() {
         window.clearTimeout(reconnectTimeout);
       }
     };
-  }, [board, boardId, isSignedIn]);
+  }, [board, boardId, isChatOpen, isPageActive, isSignedIn]);
 
   useEffect(() => {
+    if (!isPageActive) return;
     if (!isSignedIn || !boardId || !board) return;
     let isCancelled = false;
     const abortController = new AbortController();
@@ -817,7 +822,7 @@ export default function BoardDetailPage() {
         window.clearTimeout(reconnectTimeout);
       }
     };
-  }, [board, boardId, isSignedIn]);
+  }, [board, boardId, isPageActive, isSignedIn]);
 
   useEffect(() => {
     if (!selectedTask) {
@@ -840,6 +845,7 @@ export default function BoardDetailPage() {
   }, [selectedTask]);
 
   useEffect(() => {
+    if (!isPageActive) return;
     if (!isSignedIn || !boardId || !board) return;
     let isCancelled = false;
     const abortController = new AbortController();
@@ -1003,9 +1009,10 @@ export default function BoardDetailPage() {
         window.clearTimeout(reconnectTimeout);
       }
     };
-  }, [board, boardId, isSignedIn, pushLiveFeed]);
+  }, [board, boardId, isPageActive, isSignedIn, pushLiveFeed]);
 
   useEffect(() => {
+    if (!isPageActive) return;
     if (!isSignedIn || !boardId) return;
     let isCancelled = false;
     const abortController = new AbortController();
@@ -1109,7 +1116,7 @@ export default function BoardDetailPage() {
         window.clearTimeout(reconnectTimeout);
       }
     };
-  }, [board, boardId, isSignedIn]);
+  }, [board, boardId, isPageActive, isSignedIn]);
 
   const resetForm = () => {
     setTitle("");
diff --git a/frontend/src/components/BoardOnboardingChat.tsx b/frontend/src/components/BoardOnboardingChat.tsx
index 5d4e006..512cbc2 100644
--- a/frontend/src/components/BoardOnboardingChat.tsx
+++ b/frontend/src/components/BoardOnboardingChat.tsx
@@ -9,6 +9,7 @@ import {
 } from "@/components/ui/dialog";
 import { Button } from "@/components/ui/button";
 import { Textarea } from "@/components/ui/textarea";
+import { usePageActive } from "@/hooks/usePageActive";
 
 import {
   answerOnboardingApiV1BoardsBoardIdOnboardingAnswerPost,
@@ -116,6 +117,7 @@ export function BoardOnboardingChat({
   boardId: string;
   onConfirmed: (board: BoardRead) => void;
 }) {
+  const isPageActive = usePageActive();
   const [session, setSession] = useState<BoardOnboardingRead | null>(null);
   const [loading, setLoading] = useState(false);
   const [otherText, setOtherText] = useState("");
@@ -180,10 +182,15 @@ export function BoardOnboardingChat({
   }, [boardId]);
 
   useEffect(() => {
-    startSession();
+    void startSession();
+  }, [startSession]);
+
+  useEffect(() => {
+    if (!isPageActive) return;
+    void refreshSession();
     const interval = setInterval(refreshSession, 2000);
     return () => clearInterval(interval);
-  }, [startSession, refreshSession]);
+  }, [isPageActive, refreshSession]);
 
   const handleAnswer = useCallback(
     async (value: string, freeText?: string) => {
diff --git a/frontend/src/hooks/usePageActive.ts b/frontend/src/hooks/usePageActive.ts
new file mode 100644
index 0000000..1c7da12
--- /dev/null
+++ b/frontend/src/hooks/usePageActive.ts
@@ -0,0 +1,42 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+const computeIsActive = () => {
+  if (typeof document === "undefined") return true;
+  const visible =
+    document.visibilityState === "visible" &&
+    // `hidden` is a more widely-supported signal; keep both for safety.
+    !document.hidden;
+  const focused = typeof document.hasFocus === "function" ? document.hasFocus() : true;
+  return visible && focused;
+};
+
+/**
+ * Returns true when this tab/window is both visible and focused.
+ *
+ * Rationale: background tabs/windows should not keep long-lived connections
+ * (SSE/polling), otherwise opening multiple tabs can exhaust per-origin
+ * connection limits and make the app feel "hung".
+ */
+export function usePageActive(): boolean {
+  const [active, setActive] = useState<boolean>(() => computeIsActive());
+
+  useEffect(() => {
+    const update = () => setActive(computeIsActive());
+
+    update();
+    document.addEventListener("visibilitychange", update);
+    window.addEventListener("focus", update);
+    window.addEventListener("blur", update);
+
+    return () => {
+      document.removeEventListener("visibilitychange", update);
+      window.removeEventListener("focus", update);
+      window.removeEventListener("blur", update);
+    };
+  }, []);
+
+  return active;
+}
+