diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index e08896cba..4676ecb7c 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -37,6 +37,27 @@ jobs:
         if: always()
         run: python3 scripts/ruff_lint.py --all
 
+      # Static-JS runtime-error guards. These catch brick-class bugs that throw only
+      # when the browser executes the code — node --check, source-presence tests, and
+      # the mocked pytest suite all miss them. Two complementary ESLint passes:
+      #   * runtime-guard config: no-const-assign / no-import-assign (#3162 class)
+      #   * scope_undef_gate.py: no-undef across the shared classic-<script> global
+      #     scope, catching a function defined nested but called from a sibling
+      #     scope (#3696 — ReferenceError: _sessionAttentionState is not defined).
+      - name: Set up Node for ESLint runtime guards
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Install ESLint
+        run: npm install --no-save eslint@^10
+
+      - name: ESLint runtime-error gate (no-const-assign / no-import-assign, #3162)
+        run: npx eslint --no-config-lookup -c eslint.runtime-guard.config.mjs "static/**/*.js"
+
+      - name: Scope / undefined-reference gate (#3696)
+        run: python3 scripts/scope_undef_gate.py
+
   test:
     runs-on: ubuntu-latest
     strategy:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d9b2bc968..365488fe2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,13 @@
 
 ## [Unreleased]
 
+### Fixed
+- **Sidebar no longer crashes with `ReferenceError: _sessionAttentionState is not defined`.** The session-attention helper was declared *inside* `renderSessionListFromCache()` and relied on function hoisting, but the top-level `_sidebarRowHasVisibleMessages` (reached via `renderSessionListFromCache` → `_partitionSidebarSessionRows`) called it bare — and hoisting is scoped to the enclosing function, so every sidebar cache-render threw and the session list went blank. `_sessionAttentionState` is now a top-level function reachable by both call sites. Regressed in #3672 (v0.51.269). (#3696)
+- **Stale-stream terminal events no longer risk a `ReferenceError: source is not defined`.** `_bailOutOfTerminalEventsFromStaleStream` (declared inside `attachLiveStream`) called `_closeSource(source)` against a `source` that was not in its lexical scope — it would have thrown on the late-finalizing-stream path when the user is back in an active session. `source` is now threaded as an explicit parameter. Found by the new scope gate below during review. (#3696)
+
+### Internal
+- **New static-JS scope/undefined-reference gate (`scripts/scope_undef_gate.py`).** Models the WebUI's classic-`<script>` shared global scope and runs ESLint `no-undef` per file, flagging a function that is defined only *nested* but called from a sibling/top-level scope — the brick class behind #3696 that `node --check`, source-presence tests, and the existing `no-const-assign` runtime gate all miss. Wired into the CI `lint` job alongside the `no-const-assign`/`no-import-assign` runtime gate, with an in-suite test (`tests/test_static_js_scope_undef.py`) and a focused structural regression test (`tests/test_issue3696_session_attention_scope.py`). (#3696)
+
 ## [v0.51.288] — 2026-06-06 — Release JD (stage-r24 — collapsible approval card)
 
 ### Added
diff --git a/scripts/scope_undef_gate.py b/scripts/scope_undef_gate.py
new file mode 100644
index 000000000..2f12f4b91
--- /dev/null
+++ b/scripts/scope_undef_gate.py
@@ -0,0 +1,189 @@
+#!/usr/bin/env python3
+"""scope_undef_gate.py — catch the "ReferenceError: X is not defined" brick class.
+
+The WebUI front-end ships as classic (non-module) ``<script>`` tags that all share
+ONE implicit global scope. A function declared *inside* another function is NOT a
+global, so calling it (un-guarded) from a different top-level function throws
+``ReferenceError`` at runtime — but only when that code path actually executes in
+the browser. ``node --check`` (syntax only), source-presence tests, and the
+existing ``no-const-assign`` ESLint runtime gate all MISS it.
+
+Canonical bug — #3696 (v0.51.269 regression): ``_sessionAttentionState`` was
+declared *inside* ``renderSessionListFromCache()`` and relied on "function
+hoisting", but a *separate* top-level function ``_sidebarRowHasVisibleMessages``
+called it bare. Hoisting is scoped to the enclosing function, so every sidebar
+cache-render crashed with ``_sessionAttentionState is not defined`` and the
+session list went blank.
+
+How the gate works (and why it has no cross-file false positives):
+  1. It scans every static ``*.js`` file for the union of all TOP-LEVEL symbols
+     (``function NAME``, top-level ``const/let/var``, and ``window.NAME = ...``).
+     That union IS the real shared global namespace at runtime.
+  2. It lints each file individually with ESLint ``no-undef``, supplying that
+     union (plus browser/library builtins) as ``globals``. Cross-file references
+     (``api``, ``loadSession``, ``renderSessionList`` …) resolve cleanly because
+     they ARE top-level somewhere; meanwhile ESLint's per-file scope analysis
+     still flags a name that is *defined only nested* and called from a sibling
+     scope in the same file — the #3696 shape.
+  3. A short, documented allowlist covers names that are legitimately dynamic and
+     ESLint can't see are safe: helpers exposed via ``window.NAME = ...`` and
+     called bare elsewhere, and OPTIONAL helpers always called behind a
+     ``typeof NAME === 'function'`` guard (those can't throw — the guard is the
+     contract). The un-guarded bare call is exactly what makes #3696 a bug.
+
+Usage:
+  python3 scripts/scope_undef_gate.py [/path/to/webui/checkout]
+  (defaults to the repo this script lives in)
+
+Exit 0 = clean (or eslint unavailable → skip). Non-zero = a new undefined /
+scope-misplaced reference that throws at runtime. DO NOT TAG/RELEASE on non-zero.
+
+Known false-negative classes (the gate is a strong net, not a proof):
+  * Name-collision shadowing — if two files both declare a top-level symbol of the
+    same name, the union includes it, so a bare cross-scope call to that name goes
+    unflagged (runtime binds to the other file's symbol = a wrong-function bug, not
+    a ReferenceError). A duplicate top-level symbol across files is itself a smell.
+  * Non-`,;`-terminated top-level destructuring (`const {a, b} = x` where `b` ends
+    the line) is not captured by the symbol regex below. None exist in the bundle
+    today; revisit the regex if that pattern is introduced.
+  * Exposure escape hatches not scanned: `globalThis.X = X`, `Object.assign(window,
+    {X})`, `(0,eval)(...)`. None used today.
+  * The allowlist is keyed by NAME, not call-site — an entry green-lights every bare
+    reference to that name everywhere. Only add an entry when EVERY call site is a
+    `window.X =` exposure or a `typeof X === 'function'` guard; otherwise fix the
+    bug (hoist / pass the value as a param), don't allowlist it. (#3696 review:
+    a `source` allowlist entry was masking a real same-class bug at messages.js:876,
+    fixed by threading `source` as a parameter instead of allowlisting.)
+"""
+from __future__ import annotations
+
+import json
+import re
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+from shutil import which
+
+# Browser + ECMAScript builtins + CDN libraries loaded before our bundle. Explicit
+# (not an eslint "env") so the gate is self-contained and reviewable.
+BROWSER_GLOBALS = [
+    "window", "document", "console", "localStorage", "sessionStorage", "setTimeout",
+    "clearTimeout", "setInterval", "clearInterval", "requestAnimationFrame",
+    "cancelAnimationFrame", "queueMicrotask", "reportError", "fetch", "URL",
+    "URLSearchParams", "Blob", "File", "FileList", "FileReader", "FormData", "navigator",
+    "location", "history", "alert", "prompt", "confirm", "EventSource", "WebSocket",
+    "BroadcastChannel", "Image", "Audio", "MediaRecorder", "speechSynthesis",
+    "SpeechSynthesisUtterance", "AudioContext", "webkitAudioContext", "MutationObserver",
+    "IntersectionObserver", "ResizeObserver", "DataTransfer", "DragEvent", "Event",
+    "CustomEvent", "KeyboardEvent", "MouseEvent", "PointerEvent", "TouchEvent",
+    "WheelEvent", "ClipboardEvent", "getComputedStyle", "matchMedia", "atob", "btoa",
+    "structuredClone", "crypto", "performance", "screen", "DOMParser", "Node", "NodeList",
+    "HTMLElement", "Element", "Text", "AbortController", "AbortSignal", "TextDecoder",
+    "TextEncoder", "caches", "self", "CSS", "Notification", "Response", "Request",
+    "Headers", "getSelection", "scrollTo", "scrollBy", "postMessage", "indexedDB",
+    # ECMAScript builtins
+    "Promise", "Map", "Set", "WeakMap", "WeakSet", "Symbol", "Proxy", "Reflect", "JSON",
+    "Math", "Date", "RegExp", "Array", "Object", "String", "Number", "Boolean", "Error",
+    "TypeError", "RangeError", "Intl", "BigInt", "Uint8Array", "Int32Array",
+    "Float64Array", "ArrayBuffer", "DataView", "Function", "parseInt", "parseFloat",
+    "isNaN", "isFinite", "encodeURIComponent", "decodeURIComponent", "encodeURI",
+    "decodeURI", "globalThis",
+    # CDN libraries loaded via <script> before our bundle
+    "Prism", "mermaid", "katex", "hljs", "jsyaml", "Terminal", "FitAddon", "WebLinksAddon",
+]
+
+# Names ESLint can't statically prove are safe, verified NON-bugs. Each MUST be one of:
+#   (a) exposed via `window.NAME = ...` (often inside an IIFE) and called bare, or
+#   (b) an OPTIONAL helper ALWAYS called behind `typeof NAME === 'function'` (the
+#       guard means a missing binding is a no-op, never a throw), or
+#   (c) a closure variable from an enclosing function scope ESLint can't bind per-file.
+# Keep this SHORT and justified. A bare (un-guarded) call to a nested-only function is
+# a real bug and must NOT be added here — hoist the function to top level instead.
+PROJECT_DYNAMIC_GLOBALS = {
+    "placeLiveToolCardsHost": "typeof-guarded optional (ui/sessions/messages call sites)",
+    "watchInflightSession": "typeof-guarded optional fallback (sessions.js)",
+    "_applyMediaPlaybackPreferences": "typeof-guarded optional (ui.js / workspace.js)",
+}
+
+
+def _toplevel_symbols(src: str) -> set[str]:
+    syms: set[str] = set()
+    syms |= set(re.findall(r"^function\s+([A-Za-z_$][\w$]*)", src, re.M))
+    syms |= set(re.findall(r"^async\s+function\s+([A-Za-z_$][\w$]*)", src, re.M))
+    for m in re.finditer(r"^(?:const|let|var)\s+(.+)", src, re.M):
+        decl = m.group(1)
+        for name in re.findall(r"([A-Za-z_$][\w$]*)\s*[=;,]", decl):
+            syms.add(name)
+        lead = re.match(r"([A-Za-z_$][\w$]*)", decl)
+        if lead:
+            syms.add(lead.group(1))
+    syms |= set(re.findall(r"window\.([A-Za-z_$][\w$]*)\s*=", src))
+    return syms
+
+
+def main() -> int:
+    root = Path(sys.argv[1]).resolve() if len(sys.argv) > 1 else Path(__file__).resolve().parent.parent
+    static_dir = root / "static"
+    eslint = which("eslint")
+    if not eslint:
+        print("⚠ eslint not found on PATH — scope_undef_gate SKIPPED (install eslint to enable).")
+        return 0
+    if not static_dir.is_dir():
+        print(f"❌ static dir not found at {static_dir}")
+        return 2
+
+    files = [f for f in sorted(static_dir.glob("*.js")) if not f.name.endswith(".min.js")]
+    project_syms: set[str] = set()
+    for f in files:
+        project_syms |= _toplevel_symbols(f.read_text(encoding="utf-8"))
+
+    allow = project_syms | set(BROWSER_GLOBALS) | set(PROJECT_DYNAMIC_GLOBALS)
+    globals_obj = "{" + ",".join(f'"{n}":"readonly"' for n in sorted(allow)) + "}"
+
+    findings: list[tuple[str, int, str]] = []
+    with tempfile.TemporaryDirectory() as td:
+        config_path = Path(td) / "scope.config.mjs"
+        config_path.write_text(
+            "export default [{files:[\"**/*.js\"],"
+            "languageOptions:{ecmaVersion:\"latest\",sourceType:\"script\","
+            f"globals:{globals_obj}}},"
+            "rules:{\"no-undef\":\"error\"}}];",
+            encoding="utf-8",
+        )
+        for f in files:
+            proc = subprocess.run(
+                [eslint, "--no-config-lookup", "-c", str(config_path), "-f", "json", str(f)],
+                capture_output=True, text=True,
+            )
+            try:
+                report = json.loads(proc.stdout)
+            except json.JSONDecodeError:
+                print(f"❌ eslint failed on {f.name}:\n" + (proc.stderr or proc.stdout)[:1500])
+                return 2
+            for file_report in report:
+                for msg in file_report.get("messages", []):
+                    if msg.get("ruleId") == "no-undef":
+                        findings.append((f.name, msg.get("line", 0), msg.get("message", "")))
+
+    if not findings:
+        print(f"✅ scope_undef_gate: CLEAN ({len(files)} static files, "
+              f"{len(project_syms)} project globals, no undefined references).")
+        return 0
+
+    print("❌ scope_undef_gate FAILED — undefined reference(s) that throw at runtime in the "
+          "browser (brick class, see #3696):\n")
+    for src_file, line, message in findings:
+        print(f"   {src_file}:{line}: {message}")
+    print(
+        "\nA flagged name is a REAL bug when it is a function defined inside another\n"
+        "function and called BARE from a sibling/top-level scope — hoist it to top\n"
+        "level (the #3696 fix). It is only safe to add to PROJECT_DYNAMIC_GLOBALS in\n"
+        "this script (with a one-line justification) if every call site is either a\n"
+        "`window.NAME = ...` exposure or a `typeof NAME === 'function'` guard."
+    )
+    return 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/static/messages.js b/static/messages.js
index d6c78f1b3..5f890233e 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -871,7 +871,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
   function _ownsActiveStreamOrBackground(){
     return !_isActiveSession() || S.activeStreamId===streamId;
   }
-  function _bailOutOfTerminalEventsFromStaleStream(){
+  function _bailOutOfTerminalEventsFromStaleStream(source){
     if(_ownsActiveStreamOrBackground()) return false;
     _closeSource(source);
     return true;
@@ -2094,7 +2094,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
 
     source.addEventListener('done',e=>{
       if(_streamFinalized) return;
-      if(_bailOutOfTerminalEventsFromStaleStream()) return;
+      if(_bailOutOfTerminalEventsFromStaleStream(source)) return;
       // Set _streamFinalized IMMEDIATELY — before any fade delay. Without this,
       // a stream_end event arriving during the fade window sees
       // _streamFinalized=false, calls _restoreSettledSession(), and overwrites
@@ -2319,7 +2319,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
         _closeSource(source);
         return;
       }
-      if(_bailOutOfTerminalEventsFromStaleStream()) return;
+      if(_bailOutOfTerminalEventsFromStaleStream(source)) return;
       _terminalStateReached=true;
       try{
         const d=JSON.parse(e.data||'{}');
@@ -2466,7 +2466,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
     });
 
     source.addEventListener('apperror',e=>{
-      if(_bailOutOfTerminalEventsFromStaleStream()) return;
+      if(_bailOutOfTerminalEventsFromStaleStream(source)) return;
       _terminalStateReached=true;
       if(_persistTimer){clearTimeout(_persistTimer);_persistTimer=null;}
       _streamFinalized=true;
@@ -2542,7 +2542,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
     });
 
     source.addEventListener('error',async e=>{
-      if(_bailOutOfTerminalEventsFromStaleStream() && !_streamFinalized){
+      if(_bailOutOfTerminalEventsFromStaleStream(source) && !_streamFinalized){
         return;
       }
       if(_terminalStateReached || _streamFinalized){
@@ -2595,7 +2595,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
     });
 
     source.addEventListener('cancel',e=>{
-      if(_bailOutOfTerminalEventsFromStaleStream()) return;
+      if(_bailOutOfTerminalEventsFromStaleStream(source)) return;
       _terminalStateReached=true;
       if(_persistTimer){clearTimeout(_persistTimer);_persistTimer=null;}
       _streamFinalized=true;
diff --git a/static/sessions.js b/static/sessions.js
index b9373dc04..49ebad1c3 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -4170,6 +4170,29 @@ function _resyncSessionVirtualWindowAfterRender(list, expectedScrollTop, virtual
   });
 }
 
+// Top-level so BOTH the sidebar visibility predicate (_sidebarRowHasVisibleMessages,
+// reached via renderSessionListFromCache -> _partitionSidebarSessionRows) and the
+// per-row renderer (_renderOneSession, nested in renderSessionListFromCache) can call
+// it. It was previously declared INSIDE renderSessionListFromCache and relied on
+// function hoisting — but hoisting is scoped to the enclosing function, so the
+// top-level _sidebarRowHasVisibleMessages threw "ReferenceError: _sessionAttentionState
+// is not defined" on every cache render, crashing the sidebar (#3696, regressed in
+// #3672 when _sidebarRowHasVisibleMessages was extracted to top level). Pure function
+// (only its arg `s` plus the i18n global `t`), so hoisting it is safe.
+function _sessionAttentionState(s){
+  const attention=s&&s.attention&&typeof s.attention==='object'?s.attention:null;
+  if(!attention||!attention.kind||!Number.isFinite(Number(attention.count))||Number(attention.count)<=0)return null;
+  const kind=String(attention.kind)==='approval'?'approval':(String(attention.kind)==='clarify'?'clarify':'attention');
+  const count=Math.max(1,Number(attention.count)||1);
+  const labelKey=kind==='approval'?'session_attention_approval':(kind==='clarify'?'session_attention_clarify':'session_attention_generic');
+  const titleKey=kind==='approval'?'session_attention_approval_title':(kind==='clarify'?'session_attention_clarify_title':'session_attention_generic_title');
+  const fallback=kind==='approval'?(count===1?'Approval':`${count} approvals`):(kind==='clarify'?(count===1?'Question':`${count} questions`):(count===1?'Attention':`${count} items`));
+  const titleFallback=kind==='approval'?'Waiting for permission decision':(kind==='clarify'?'Waiting for your answer':'Waiting for user action');
+  const label=(typeof t==='function')?t(labelKey,count):fallback;
+  const title=(typeof t==='function')?t(titleKey,count):titleFallback;
+  return {kind,count,severity:String(attention.severity||''),label,title};
+}
+
 function _sidebarRowHasVisibleMessages(s, activeSidForSidebar){
   return (s.message_count||0)>0 ||
     _sessionAttentionState(s) ||
@@ -4523,20 +4546,6 @@ function renderSessionListFromCache(){
   const reflowTimeout=animateRefresh?SESSION_LIST_FLIP_TIMEOUT_MS:SESSION_REFLOW_TIMEOUT_MS;
   _pendingSessionReflowPositions=null;
   _playSessionRowsReflowFromPositions(reflowBefore,reflowTimeout,_sessionPrefersReducedMotion);
-  // Note: declared after the groups loop but available via function hoisting.
-  function _sessionAttentionState(s){
-    const attention=s&&s.attention&&typeof s.attention==='object'?s.attention:null;
-    if(!attention||!attention.kind||!Number.isFinite(Number(attention.count))||Number(attention.count)<=0)return null;
-    const kind=String(attention.kind)==='approval'?'approval':(String(attention.kind)==='clarify'?'clarify':'attention');
-    const count=Math.max(1,Number(attention.count)||1);
-    const labelKey=kind==='approval'?'session_attention_approval':(kind==='clarify'?'session_attention_clarify':'session_attention_generic');
-    const titleKey=kind==='approval'?'session_attention_approval_title':(kind==='clarify'?'session_attention_clarify_title':'session_attention_generic_title');
-    const fallback=kind==='approval'?(count===1?'Approval':`${count} approvals`):(kind==='clarify'?(count===1?'Question':`${count} questions`):(count===1?'Attention':`${count} items`));
-    const titleFallback=kind==='approval'?'Waiting for permission decision':(kind==='clarify'?'Waiting for your answer':'Waiting for user action');
-    const label=(typeof t==='function')?t(labelKey,count):fallback;
-    const title=(typeof t==='function')?t(titleKey,count):titleFallback;
-    return {kind,count,severity:String(attention.severity||''),label,title};
-  }
 
   function _renderOneSession(s, isPinnedGroup=false){
     const el=document.createElement('div');
diff --git a/tests/test_issue3696_session_attention_scope.py b/tests/test_issue3696_session_attention_scope.py
new file mode 100644
index 000000000..dbc8e44c1
--- /dev/null
+++ b/tests/test_issue3696_session_attention_scope.py
@@ -0,0 +1,83 @@
+"""Regression test for #3696 — `_sessionAttentionState is not defined`.
+
+Bug: `_sessionAttentionState` was declared INSIDE `renderSessionListFromCache()`
+and relied on "function hoisting", but the separate top-level function
+`_sidebarRowHasVisibleMessages` (reached via renderSessionListFromCache ->
+_partitionSidebarSessionRows) called it BARE. Function hoisting is scoped to the
+enclosing function, so the call threw `ReferenceError: _sessionAttentionState is
+not defined` on every sidebar cache-render — the session list went blank
+(v0.51.269, regressed by #3672 when _sidebarRowHasVisibleMessages was extracted
+to top level).
+
+Fix: hoist `_sessionAttentionState` to top-level (module/global) scope so both the
+top-level visibility predicate and the nested per-row renderer can reach it.
+
+This is a structural test (no node/eslint needed, runs in every shard). The
+behavioral scope-analysis guard lives in tests/test_static_js_scope_undef.py +
+scripts/scope_undef_gate.py, which catch the whole class. This test pins the
+specific #3696 invariant cheaply.
+"""
+import re
+from pathlib import Path
+
+SESSIONS_JS = (Path(__file__).resolve().parents[1] / "static" / "sessions.js").read_text(encoding="utf-8")
+
+
+def _brace_body(src: str, open_brace_idx: int) -> tuple[int, int]:
+    """Return (start, end) char offsets of the body delimited by the brace at
+    open_brace_idx (exclusive of the braces)."""
+    depth = 1
+    i = open_brace_idx + 1
+    while i < len(src) and depth:
+        if src[i] == "{":
+            depth += 1
+        elif src[i] == "}":
+            depth -= 1
+        i += 1
+    return open_brace_idx + 1, i - 1
+
+
+def _function_span(src: str, name: str) -> tuple[int, int]:
+    m = re.search(r"function\s+" + re.escape(name) + r"\s*\(", src)
+    assert m, f"function {name} not found"
+    brace = src.find("{", m.end())
+    return _brace_body(src, brace)
+
+
+def test_session_attention_state_is_top_level():
+    """`_sessionAttentionState` must be declared at top-level scope (column 0),
+    not nested inside another function — otherwise the top-level callers throw
+    ReferenceError (#3696)."""
+    decls = re.findall(r"^(\s*)function\s+_sessionAttentionState\s*\(", SESSIONS_JS, re.M)
+    assert decls, "_sessionAttentionState declaration not found"
+    assert len(decls) == 1, f"expected exactly one declaration, found {len(decls)}"
+    assert decls[0] == "", (
+        "#3696: _sessionAttentionState must be a TOP-LEVEL function (no leading "
+        f"indentation), but it is indented {len(decls[0])} spaces (nested). A nested "
+        "declaration only hoists within its enclosing function, so top-level callers "
+        "like _sidebarRowHasVisibleMessages throw 'ReferenceError: _sessionAttentionState "
+        "is not defined'."
+    )
+
+
+def test_session_attention_state_not_nested_in_render_from_cache():
+    """Belt-and-suspenders: the definition must NOT live inside the body of
+    `renderSessionListFromCache` (where it was when #3696 shipped)."""
+    start, end = _function_span(SESSIONS_JS, "renderSessionListFromCache")
+    body = SESSIONS_JS[start:end]
+    assert "function _sessionAttentionState(" not in body, (
+        "#3696: _sessionAttentionState is defined inside renderSessionListFromCache() — "
+        "it must be hoisted to top-level so _sidebarRowHasVisibleMessages can call it."
+    )
+
+
+def test_sidebar_visibility_predicate_calls_attention_state():
+    """Guard the regression's trigger: _sidebarRowHasVisibleMessages (top-level)
+    references _sessionAttentionState. If this call is ever removed the bug can't
+    recur, but while it exists the function MUST be top-level (asserted above)."""
+    start, end = _function_span(SESSIONS_JS, "_sidebarRowHasVisibleMessages")
+    body = SESSIONS_JS[start:end]
+    assert "_sessionAttentionState(" in body, (
+        "_sidebarRowHasVisibleMessages no longer calls _sessionAttentionState — if this "
+        "is intentional, update this test; the #3696 invariant assumes this call exists."
+    )
diff --git a/tests/test_messages_stale_stream_source_scope.py b/tests/test_messages_stale_stream_source_scope.py
new file mode 100644
index 000000000..0c64df8ae
--- /dev/null
+++ b/tests/test_messages_stale_stream_source_scope.py
@@ -0,0 +1,56 @@
+"""Regression test for the messages.js stale-stream `source` scope bug.
+
+Surfaced by the scope_undef_gate (scripts/scope_undef_gate.py) during the #3696
+review: `_bailOutOfTerminalEventsFromStaleStream` is declared at brace depth 2
+inside `attachLiveStream` (whose params are activeSid/streamId/uploaded/options —
+no `source`), yet its body called `_closeSource(source)` referencing a `source`
+that is NOT in its lexical scope. All call sites live inside `_wireSSE(source)`,
+but JS scoping is lexical not dynamic, so when the helper runs it would throw
+`ReferenceError: source is not defined` on the stale-stream terminal-event path
+(`_ownsActiveStreamOrBackground()` false → user back in an active session whose
+old stream finalizes late). Same class as #3696.
+
+Fix: thread `source` as an explicit parameter
+(`_bailOutOfTerminalEventsFromStaleStream(source)`) and pass it at every call
+site, instead of relying on (broken) scope resolution. This test locks that the
+declaration takes the param and no bare-call site remains.
+"""
+import re
+from pathlib import Path
+
+MESSAGES_JS = (Path(__file__).resolve().parents[1] / "static" / "messages.js").read_text(encoding="utf-8")
+
+
+def test_bailout_helper_takes_source_param():
+    """The helper must declare a `source` parameter (not rely on an out-of-scope
+    closure variable)."""
+    m = re.search(r"function\s+_bailOutOfTerminalEventsFromStaleStream\s*\(([^)]*)\)", MESSAGES_JS)
+    assert m, "_bailOutOfTerminalEventsFromStaleStream declaration not found"
+    params = [p.strip() for p in m.group(1).split(",") if p.strip()]
+    assert "source" in params, (
+        "_bailOutOfTerminalEventsFromStaleStream must take `source` as a parameter — "
+        "it calls _closeSource(source) but is declared inside attachLiveStream (no "
+        "`source` in scope), so a bare reference throws ReferenceError on the "
+        f"stale-stream path. Current params: {params}"
+    )
+
+
+def test_no_bare_bailout_call_sites_remain():
+    """Every call site must pass `source` — a bare `()` call would leave the helper's
+    `source` undefined again."""
+    bare = re.findall(r"_bailOutOfTerminalEventsFromStaleStream\(\s*\)", MESSAGES_JS)
+    assert not bare, (
+        f"Found {len(bare)} bare _bailOutOfTerminalEventsFromStaleStream() call site(s) "
+        "with no argument — each must pass `source` so the helper can close the right "
+        "stream. A bare call reintroduces the ReferenceError."
+    )
+
+
+def test_bailout_call_sites_pass_source():
+    """Positive check: the call sites pass `source`."""
+    calls = re.findall(r"_bailOutOfTerminalEventsFromStaleStream\(\s*source\s*\)", MESSAGES_JS)
+    assert len(calls) >= 5, (
+        f"expected >=5 call sites passing `source`, found {len(calls)} — if the SSE "
+        "terminal-event wiring changed, update this count, but every call must still "
+        "pass source."
+    )
diff --git a/tests/test_profile_switch_ux.py b/tests/test_profile_switch_ux.py
index 9dbe65977..723ca96fa 100644
--- a/tests/test_profile_switch_ux.py
+++ b/tests/test_profile_switch_ux.py
@@ -178,7 +178,11 @@ class TestProfileSessionListFlip:
 
     def test_profile_refresh_drops_queued_reflow_before_playing_flip(self):
         start = self.JS.index("// Refresh FLIP and queued archive/delete reflow both drive")
-        end = self.JS.index("// Note: declared after the groups loop", start)
+        # End anchor: the next function declaration after the reflow block. (Was the
+        # "// Note: declared after the groups loop" comment on the nested
+        # _sessionAttentionState, which #3696 removed when that helper was hoisted to
+        # top-level scope — so anchor on the stable _renderOneSession decl instead.)
+        end = self.JS.index("function _renderOneSession(", start)
         block = self.JS[start:end]
 
         assert "const reflowBefore=animateRefresh?flipBefore:_pendingSessionReflowPositions;" in block
diff --git a/tests/test_static_js_scope_undef.py b/tests/test_static_js_scope_undef.py
new file mode 100644
index 000000000..07c446718
--- /dev/null
+++ b/tests/test_static_js_scope_undef.py
@@ -0,0 +1,58 @@
+"""Scope / undefined-reference guard for the static JS bundle (issue #3696).
+
+Why this exists: #3696 was a brick-class regression — `_sessionAttentionState`
+was declared *inside* `renderSessionListFromCache()` but called (un-guarded) from
+a separate top-level function `_sidebarRowHasVisibleMessages`. Function hoisting is
+scoped to the enclosing function, so the call threw
+`ReferenceError: _sessionAttentionState is not defined` on every sidebar
+cache-render and the session list went blank (v0.51.269, regressed by #3672).
+
+Nothing caught it: `node --check` is a syntax check (a nested function IS valid
+syntax), source-presence tests asserted the strings existed (they did — in the
+wrong scope), and the existing `no-const-assign` runtime gate only covers
+const-reassign / import-assign. This is a DIFFERENT runtime-error class:
+referencing a name that isn't in scope.
+
+`scripts/scope_undef_gate.py` models the WebUI's classic-`<script>` shared global
+scope (all top-level symbols across every static file become one namespace), then
+runs ESLint `no-undef` per file. Cross-file globals resolve; a function defined
+only nested and called from a sibling scope is flagged. See that script's header
+for the full design + the verified dynamic-global allowlist.
+
+Graceful skip: if node or eslint isn't available the test SKIPS (the release gate
+runs where eslint IS installed — see TESTING.md), so toolchain-free envs aren't
+blocked.
+"""
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+REPO = Path(__file__).resolve().parents[1]
+GATE = REPO / "scripts" / "scope_undef_gate.py"
+
+
+@pytest.mark.skipif(not GATE.exists(), reason="scope_undef_gate.py missing")
+def test_static_js_has_no_undefined_references():
+    if shutil.which("eslint") is None:
+        pytest.skip(
+            "eslint not installed — install with "
+            "`npm install --no-save --before=<48h-ago> eslint` to enforce the "
+            "scope/undef guard locally (CI/release env has it). See TESTING.md."
+        )
+    if shutil.which("node") is None:
+        pytest.skip("node not available")
+
+    proc = subprocess.run(
+        [sys.executable, str(GATE), str(REPO)],
+        capture_output=True, text=True, timeout=180,
+    )
+    # Exit 0 = clean or eslint-skip; 1 = real finding; 2 = setup error.
+    assert proc.returncode != 1, (
+        "scope_undef_gate found undefined reference(s) that throw at runtime in the "
+        "browser (brick-class, see #3696):\n" + proc.stdout + proc.stderr
+    )
+    if proc.returncode == 2:
+        pytest.skip(f"scope_undef_gate setup issue (not a code failure): {proc.stdout[:300]}")