diff --git a/CHANGELOG.md b/CHANGELOG.md
index 15240b16..33f43d15 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **Multiple PWA windows no longer saturate the connection pool and cycle "Request timed out" toasts (#4151).** The idle-SSE close added in #3992/#3996 keyed off the Page Visibility API (`visibilitychange` / `document.hidden`), but a PWA *standalone* window does not reliably fire `visibilitychange` when it loses focus to another window of the same app — `document.hidden` only flips on minimize. So two side-by-side PWA windows both stayed `visible`, each held its two global sidebar SSE streams open (session-events + gateway), and 2×3 = 6 connections hit the browser's per-origin HTTP/1.1 limit; subsequent `fetch()` calls (the 30s background polls) queued behind the saturated pool and timed out. The two global sidebar streams now also close on a sustained window `blur` (gated on `document.hasFocus()`, the signal `visibilitychange` misses) and reopen — catching up the session list — on `focus`. The per-session live stream is intentionally left visibility-only so an unfocused-but-visible window still receives live `bg_task_complete` / `server_turn_started` events. (#4151)
+
 ## [v0.51.403] — 2026-06-13 — Release NP (notification click reuses the existing chat tab, #4109)
 
 ### Fixed
diff --git a/static/sessions.js b/static/sessions.js
index b31cf681..ab416550 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -3645,6 +3645,62 @@ function _scheduleSessionEventsRefresh(reason){
   }, 300);
 }
 
+// ── #4151: focus-aware close for the two GLOBAL sidebar SSE streams ──────────
+// Each WebUI window holds up to three persistent SSE connections (session-events
+// + gateway + the per-session stream). #3992/#3996 close them on the Page
+// Visibility API (`visibilitychange` / `document.hidden`) so a hidden tab frees
+// HTTP/1.1 pool slots. But a PWA *standalone* window does NOT reliably fire
+// `visibilitychange` when it merely loses focus to another window of the same
+// app — `document.hidden` only flips on minimize. So two side-by-side PWA windows
+// both stay `visibilityState==='visible'`, each keeps its sidebar streams open,
+// and 2x3 = 6 = the per-origin HTTP/1.1 connection limit; every later fetch()
+// (the 30s polls) queues behind the saturated pool and times out (#4151).
+// `document.hasFocus()` is the signal `visibilitychange` misses — only one window
+// holds focus at a time.
+//
+// Scope: ONLY the two global sidebar streams (session-events + gateway). The
+// per-session live stream (messages.js `startSessionStream`) deliberately stays
+// visibility-only — it carries live `bg_task_complete` toasts and
+// `server_turn_started` live-view that an unfocused-but-VISIBLE window must still
+// receive (the OS-notification path is gated on `document.hidden`, so the in-app
+// toast is the only completion signal a visible-unfocused window gets). Closing
+// it on blur would regress the multi-window live-view UX.
+function _sidebarSseBackgrounded(){
+  if(typeof document === 'undefined') return false;
+  if(document.hidden) return true;
+  if(typeof document.hasFocus === 'function' && !document.hasFocus()) return true;
+  return false;
+}
+
+let _sidebarSseBlurCloseTimer = 0;
+// Debounce the blur-close so a transient blur (native dialog, quick alt-tab and
+// back) doesn't thrash the streams; a sustained blur frees the pool slots.
+const _SIDEBAR_SSE_BLUR_CLOSE_MS = 1000;
+
+function _installSidebarSseFocusHook(){
+  if(typeof window === 'undefined' || typeof document === 'undefined') return;
+  if(document._hermesSidebarSseFocusHook) return;
+  document._hermesSidebarSseFocusHook = true;
+  window.addEventListener('blur', () => {
+    if(_sidebarSseBlurCloseTimer) return;
+    _sidebarSseBlurCloseTimer = setTimeout(() => {
+      _sidebarSseBlurCloseTimer = 0;
+      // Re-check at fire time — focus may have returned during the debounce.
+      if(_sidebarSseBackgrounded()){
+        _closeSessionEventsSSE();
+        stopGatewaySSE();
+      }
+    }, _SIDEBAR_SSE_BLUR_CLOSE_MS);
+  });
+  window.addEventListener('focus', () => {
+    if(_sidebarSseBlurCloseTimer){ clearTimeout(_sidebarSseBlurCloseTimer); _sidebarSseBlurCloseTimer = 0; }
+    // Reopen (both ensure-idempotent) and catch up on anything missed while blurred.
+    ensureSessionEventsSSE();
+    startGatewaySSE();
+    void refreshSessionList('focus');
+  });
+}
+
 function _closeSessionEventsSSE(){
   if(_sessionEventsSSE){
     _sessionEventsSSE.close();
@@ -3664,8 +3720,9 @@ function ensureSessionEventsSSE(){
     });
     document._hermesSessionEventsVisibilityHook = true;
   }
+  _installSidebarSseFocusHook();
   if(typeof EventSource==='undefined') return;
-  if(typeof document !== 'undefined' && document.hidden) return;
+  if(_sidebarSseBackgrounded()) return;
   if(_sessionEventsSSE) return;
   try{
     // Same-origin relative URL preserves subpath mounts and normal WebUI cookies.
@@ -3788,8 +3845,10 @@ function startGatewaySSE(){
     });
     document._hermesGatewaySSEVisibilityHook = true;
   }
-  // Don't open when tab is hidden — saves connection pool slots
-  if(typeof document !== 'undefined' && document.hidden) return;
+  _installSidebarSseFocusHook();
+  // Don't open when tab is hidden OR the window has lost focus (PWA blur) —
+  // saves connection pool slots (#4151).
+  if(_sidebarSseBackgrounded()) return;
   try{
     _gatewaySSE = new EventSource('api/sessions/gateway/stream');
     _gatewaySSE.addEventListener('sessions_changed', (ev) => {
diff --git a/tests/test_issue4151_pwa_focus_sse.py b/tests/test_issue4151_pwa_focus_sse.py
new file mode 100644
index 00000000..eb385cbb
--- /dev/null
+++ b/tests/test_issue4151_pwa_focus_sse.py
@@ -0,0 +1,135 @@
+"""Structural tests for #4151 — PWA two-window connection-pool saturation.
+
+#3992/#3996 close the idle SSE streams on the Page Visibility API
+(`visibilitychange` / `document.hidden`). But a PWA *standalone* window does NOT
+reliably fire `visibilitychange` when it loses focus to another window of the
+same app — `document.hidden` only flips on minimize. So two side-by-side PWA
+windows both stay `visibilityState==='visible'`, each holds its sidebar SSE
+streams open, and 2x3 = 6 = the per-origin HTTP/1.1 connection limit; every
+later fetch() queues behind the saturated pool and times out (#4151).
+
+The fix makes the two GLOBAL sidebar streams (session-events + gateway) also
+close on window `blur` (gated on `document.hasFocus()`, the signal
+`visibilitychange` misses) and reopen on `focus`, via a shared
+`_sidebarSseBackgrounded()` predicate and a debounced `_installSidebarSseFocusHook()`.
+
+CRITICAL SCOPE GUARD (the regression these tests lock): the PER-SESSION live
+stream (`startSessionStream` in messages.js) must stay visibility-only and must
+NOT be torn down on blur — it carries live `bg_task_complete` toasts +
+`server_turn_started` live-view that an unfocused-but-VISIBLE window must still
+receive. So the focus hook lives in sessions.js and only touches
+`_closeSessionEventsSSE()` + `stopGatewaySSE()`.
+
+Source-grep checks (the hooks live in static JS with no server round trip).
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+MESSAGES_JS = (REPO_ROOT / "static" / "messages.js").read_text(encoding="utf-8")
+SESSIONS_JS = (REPO_ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
+
+
+def test_backgrounded_predicate_uses_hasfocus_not_only_hidden():
+    """_sidebarSseBackgrounded() must consult document.hasFocus(), not only document.hidden.
+
+    document.hidden alone is exactly what misses the PWA blur case; the predicate
+    has to treat a visible-but-unfocused window as backgrounded.
+    """
+    assert "function _sidebarSseBackgrounded()" in SESSIONS_JS
+    start = SESSIONS_JS.find("function _sidebarSseBackgrounded()")
+    block = SESSIONS_JS[start:start + 320]
+    assert "document.hidden" in block
+    assert "document.hasFocus" in block
+    assert "!document.hasFocus()" in block
+
+
+def test_focus_hook_closes_both_global_sidebar_streams():
+    """The blur path closes the session-events AND gateway streams."""
+    assert "function _installSidebarSseFocusHook()" in SESSIONS_JS
+    start = SESSIONS_JS.find("function _installSidebarSseFocusHook()")
+    block = SESSIONS_JS[start:start + 1100]
+    # Installed once.
+    assert "_hermesSidebarSseFocusHook" in block
+    # Blur listener tears down both global streams.
+    assert "window.addEventListener('blur'" in block
+    assert "_closeSessionEventsSSE()" in block
+    assert "stopGatewaySSE()" in block
+    # Focus listener reopens both and refreshes the list.
+    assert "window.addEventListener('focus'" in block
+    assert "ensureSessionEventsSSE()" in block
+    assert "startGatewaySSE()" in block
+
+
+def test_blur_close_is_debounced_and_rechecks_focus_at_fire_time():
+    """A transient blur must not thrash the streams.
+
+    The blur close is scheduled on a timer and re-checks _sidebarSseBackgrounded()
+    when it fires, so focus returning during the debounce cancels the teardown.
+    """
+    start = SESSIONS_JS.find("function _installSidebarSseFocusHook()")
+    block = SESSIONS_JS[start:start + 1100]
+    assert "_sidebarSseBlurCloseTimer" in block
+    assert "setTimeout(" in block
+    # Re-check guards the actual close so a returned focus is a no-op.
+    assert "if(_sidebarSseBackgrounded()){" in block
+    # Focus listener clears any pending blur-close timer.
+    assert "clearTimeout(_sidebarSseBlurCloseTimer)" in block
+
+
+def test_session_events_open_guard_uses_backgrounded_predicate():
+    """ensureSessionEventsSSE installs the focus hook and gates open on the predicate."""
+    start = SESSIONS_JS.find("function ensureSessionEventsSSE()")
+    assert start != -1
+    block = SESSIONS_JS[start:start + 700]
+    assert "_installSidebarSseFocusHook()" in block
+    # Open guard is the focus-aware predicate, not the old hidden-only check.
+    assert "if(_sidebarSseBackgrounded()) return;" in block
+
+
+def test_gateway_open_guard_uses_backgrounded_predicate():
+    """startGatewaySSE installs the focus hook and gates open on the predicate."""
+    start = SESSIONS_JS.find("function startGatewaySSE()")
+    assert start != -1
+    block = SESSIONS_JS[start:start + 700]
+    assert "_installSidebarSseFocusHook()" in block
+    assert "if(_sidebarSseBackgrounded()) return;" in block
+
+
+def test_per_session_stream_NOT_closed_on_blur():
+    """REGRESSION GUARD: the per-session live stream must stay visibility-only.
+
+    startSessionStream carries live bg_task_complete toasts + server_turn_started
+    live-view that an unfocused-but-visible window must still get. The focus hook
+    must NOT tear it down, so:
+      (a) messages.js must not add a window 'blur' listener that calls stopSessionStream, and
+      (b) the focus hook in sessions.js must not reference stopSessionStream/startSessionStream.
+    """
+    # (a) the per-session stream file must not tear down the stream on blur.
+    #     (A pre-existing composer speech-synthesis blur handler on _msgEl is
+    #     fine — the guard is specifically that no blur path calls
+    #     stopSessionStream.) Scan each blur-listener body in messages.js.
+    import re
+    for m in re.finditer(r"addEventListener\(\s*['\"]blur['\"]", MESSAGES_JS):
+        tail = MESSAGES_JS[m.start():m.start() + 200]
+        assert "stopSessionStream" not in tail, (
+            "a blur listener in messages.js tears down the per-session stream — "
+            "that regresses live bg_task_complete / server_turn_started for an "
+            "unfocused-but-visible window (#4151 scope guard)"
+        )
+    # (b) the sidebar focus hook only manages the two global streams.
+    start = SESSIONS_JS.find("function _installSidebarSseFocusHook()")
+    block = SESSIONS_JS[start:start + 1100]
+    assert "stopSessionStream" not in block
+    assert "startSessionStream" not in block
+
+
+def test_per_session_stream_still_visibility_gated():
+    """Sanity: the per-session stream keeps its existing visibility hook untouched."""
+    assert "_hermesSessionStreamVisibilityHook" in MESSAGES_JS
+    start = MESSAGES_JS.find("function startSessionStream(sid)")
+    block = MESSAGES_JS[start:start + 1700]
+    assert "visibilitychange" in block
+    assert "document.hidden" in block
diff --git a/tests/test_webui_external_refresh_frontend.py b/tests/test_webui_external_refresh_frontend.py
index b8ce081d..cc59d30c 100644
--- a/tests/test_webui_external_refresh_frontend.py
+++ b/tests/test_webui_external_refresh_frontend.py
@@ -47,7 +47,10 @@ def test_session_list_external_refresh_uses_sse_invalidation_not_polling():
     assert "ensureSessionEventsSSE();" in SESSIONS_JS
     assert "document._hermesSessionEventsVisibilityHook" in SESSIONS_JS
     ensure_fn = SESSIONS_JS[SESSIONS_JS.find("function ensureSessionEventsSSE()") :]
-    assert ensure_fn.find("document._hermesSessionEventsVisibilityHook") < ensure_fn.find("document.hidden) return")
+    # The visibility hook must be installed before the open-guard early-return.
+    # #4151 replaced the `document.hidden) return` open guard with the focus-aware
+    # `_sidebarSseBackgrounded()) return` predicate (which also covers PWA blur).
+    assert ensure_fn.find("document._hermesSessionEventsVisibilityHook") < ensure_fn.find("_sidebarSseBackgrounded()) return")
     assert "_sessionListExternalRefreshMs" not in SESSIONS_JS
     assert "addEventListener('sessions_changed', (ev) => {" in ensure_fn
     assert "const activeProfile = S.activeProfile || 'default';" in ensure_fn