diff --git a/frontend/live-data-status.js b/frontend/live-data-status.js
index 396ff3d..328b966 100644
--- a/frontend/live-data-status.js
+++ b/frontend/live-data-status.js
@@ -15,8 +15,14 @@
   }
 
   function boundedLatencyMs(value) {
-    const ms = Number(value);
-    return Number.isFinite(ms) && ms >= 0 ? Math.min(Math.round(ms), 3600000) : null;
+    // Strict acceptance only: actual finite nonnegative bounded integers.
+    // Rejects null, booleans, strings, arrays, floats, fractions, negatives,
+    // Infinity, and huge values. No Number() coercion, no clipping.
+    if (typeof value !== 'number') return null;
+    if (!Number.isFinite(value)) return null;
+    if (!Number.isInteger(value)) return null;
+    if (value < 0 || value > 3600000) return null;
+    return value;
   }
 
   const latencyBuckets = [
@@ -67,12 +73,24 @@
 
   function formatLatency(ms) {
     if (ms === null) return 'not measured';
+    if (ms === 0) return '<1 ms';
     if (ms >= 10000) return Math.round(ms / 1000) + ' s';
     if (ms >= 1000) return (ms / 1000).toFixed(1) + ' s';
     return ms + ' ms';
   }
 
   function renderBucketLatency(description, panel) {
+    // Render rows *inside* the existing #bucket-latency-rows container so the
+    // panel heading and its aria-labelledby target are never replaced. Each
+    // generated row carries role=listitem so the list semantics hold.
+    const rowsContainer = (typeof panel.querySelector === 'function')
+      ? (panel.querySelector('#bucket-latency-rows') || panel)
+      : panel;
+    if (rowsContainer !== panel) {
+      rowsContainer.innerHTML = '';
+    } else {
+      panel.innerHTML = '';
+    }
     const rows = description.feeds.map(feed => ({
       key: feed.key,
       label: feed.label,
@@ -81,20 +99,26 @@
     }));
     if (!rows.some(row => row.latencyMs !== null)) {
       panel.hidden = true;
-      panel.innerHTML = '';
       return rows;
     }
     panel.hidden = false;
-    panel.innerHTML = rows.map(row => {
+    for (const row of rows) {
       const slow = row.latencyMs !== null && row.latencyMs >= 2000 ? ' slow' : '';
       const value = formatLatency(row.latencyMs);
-      // A delayed feed shows the last measurement that succeeded, so say so.
-      const suffix = row.stale && row.latencyMs !== null ? ' <span class="small muted">last known</span>' : '';
-      return '<div class="bucket-latency-row" data-bucket="' + row.key + '">' +
-        '<strong>' + row.label + '</strong>' +
-        '<span class="bucket-latency-value' + slow + '">' + value + suffix + '</span>' +
-        '</div>';
-    }).join('');
+      // A stale feed shows the last measurement that succeeded, so say so.
+      const suffix = row.stale && row.latencyMs !== null
+        ? ' <span class="small muted">last known</span>' : '';
+      const html = '<div class="bucket-latency-row' + slow + '" role="listitem" data-bucket="' +
+        row.key + '"><strong>' + row.label + '</strong>' +
+        '<span class="bucket-latency-value' + slow + '">' + value + suffix + '</span></div>';
+      if (typeof document !== 'undefined') {
+        const wrapper = document.createElement('div');
+        wrapper.innerHTML = html;
+        rowsContainer.appendChild(wrapper.firstChild);
+      } else {
+        rowsContainer.appendChild({outerHTML: html});
+      }
+    }
     return rows;
   }
 
diff --git a/src/main.py b/src/main.py
index 4a58463..97abeed 100644
--- a/src/main.py
+++ b/src/main.py
@@ -329,6 +329,46 @@ BULK_NOTIFICATION_DEADLINE_SECONDS = 6.0
 LIVE_SNAPSHOT_FRESHNESS_SECONDS = 8.0
 LIVE_SNAPSHOT_RETRY_BASE_SECONDS = 5.0
 LIVE_SNAPSHOT_RETRY_MAX_SECONDS = 60.0
+LIVE_SNAPSHOT_LATENCY_MAX_MS = 3_600_000
+
+
+def _measured_latency_ms(elapsed_seconds: float) -> int | None:
+    """Convert a measured elapsed duration to a strict latency integer.
+
+    Resolution is honest: sub-millisecond work yields 0 (rendered client-side
+    as '<1 ms'), never a fabricated 1 ms floor. Only finite, nonnegative,
+    bounded integer milliseconds are returned; any non-finite or out-of-range
+    measurement is rejected as None so malformed values are never coerced or
+    clipped into a measurement.
+    """
+    if not math.isfinite(elapsed_seconds) or elapsed_seconds < 0:
+        return None
+    ms = round(elapsed_seconds * 1000)
+    if not isinstance(ms, int) or isinstance(ms, bool):
+        return None
+    if ms < 0 or ms > LIVE_SNAPSHOT_LATENCY_MAX_MS:
+        return None
+    return ms
+
+
+def _valid_latency_ms(value: Any) -> bool:
+    """Strict check for a valid telemetry latency integer.
+
+    Accepts only actual finite nonnegative bounded integers. Rejects None,
+    booleans, strings, arrays, floats, fractions, negatives, Infinity, and
+    huge values. Malformed values are never coerced or clipped.
+    """
+    if value is None:
+        return False
+    if isinstance(value, bool):
+        return False
+    if not isinstance(value, int):
+        return False
+    if not math.isfinite(value):  # int is always finite, but be defensive
+        return False
+    if value < 0 or value > LIVE_SNAPSHOT_LATENCY_MAX_MS:
+        return False
+    return True
 AVAILABLE_ISSUE_SNAPSHOT_FRESHNESS_SECONDS = 15.0
 AVAILABLE_ISSUE_SNAPSHOT_RETRY_SECONDS = 5.0
 AVAILABLE_ISSUE_SNAPSHOT_LEASE_SECONDS = WORK_PAGE_TIMEOUT_SECONDS + 1.0
@@ -4047,48 +4087,56 @@ async def _build_live_snapshot(sections: set[str] | None = None) -> dict:
     results: dict[str, object] = {}
     user_data: dict | None = None
     latency_ms: dict[str, int] = {}
-    # The shared current_user() fetch is a real upstream cost of producing the
-    # Work and Activity feeds, so its measured duration counts toward both.
+    # Notifications is auth-independent and may start on the same tick as the
+    # shared current_user() fetch; Work (context) and Activity (events) await
+    # that shared auth, then run their dependent calls. The auth duration is a
+    # real upstream cost attributed to both the Work and Activity buckets.
     auth_sections = requested & {"context", "events"}
     auth_start = perf_counter()
+    auth_result: dict | BaseException | None = None
+    notifications_task: asyncio.Task | None = None
+
+    async def _timed_load(section: str, awaitable: Awaitable[Any]) -> Any:
+        start = perf_counter()
+        try:
+            outcome = await awaitable
+        except BaseException as exc:
+            return exc
+        measured = _measured_latency_ms(perf_counter() - start)
+        if measured is not None:
+            latency_ms[section] = latency_ms.get(section, 0) + measured
+        return outcome
+
+    if "notifications" in requested:
+        notifications_task = asyncio.create_task(_timed_load("notifications", notifications()))
+
     if auth_sections:
         try:
             user_data = await current_user()
             if not isinstance(user_data, dict) or not user_data.get("login"):
                 raise ContextPayloadError("Gitea current-user response was invalid")
+            auth_result = user_data
+            auth_ms = _measured_latency_ms(perf_counter() - auth_start)
+            if auth_ms is not None:
+                for section in auth_sections:
+                    latency_ms[section] = latency_ms.get(section, 0) + auth_ms
         except Exception as exc:
+            auth_result = exc
             for section in auth_sections:
                 results[section] = exc
-        finally:
-            auth_ms = max(1, round((perf_counter() - auth_start) * 1000))
-            for section in auth_sections:
-                latency_ms[section] = latency_ms.get(section, 0) + auth_ms
+    else:
+        auth_result = None
 
     loads: dict[str, Awaitable[Any]] = {}
-    if "context" in requested and "context" not in results:
-        assert user_data is not None
-        loads["context"] = _load_context_for_user(user_data)
-    if "events" in requested and "events" not in results:
-        assert user_data is not None
-        loads["events"] = activity_events(user_data)
-    if "notifications" in requested:
-        loads["notifications"] = notifications()
+    if "context" in requested and "context" not in results and isinstance(auth_result, dict):
+        loads["context"] = _timed_load("context", _load_context_for_user(auth_result))
+    if "events" in requested and "events" not in results and isinstance(auth_result, dict):
+        loads["events"] = _timed_load("events", activity_events(auth_result))
     if loads:
-        async def _timed(section: str, awaitable: Awaitable[Any]) -> Any:
-            start = perf_counter()
-            try:
-                return await awaitable
-            finally:
-                latency_ms[section] = (
-                    latency_ms.get(section, 0)
-                    + max(1, round((perf_counter() - start) * 1000))
-                )
-
-        loaded = await asyncio.gather(
-            *(_timed(section, awaitable) for section, awaitable in loads.items()),
-            return_exceptions=True,
-        )
+        loaded = await asyncio.gather(*loads.values(), return_exceptions=True)
         results.update(zip(loads, loaded))
+    if notifications_task is not None:
+        results["notifications"] = await notifications_task
 
     context_result = results.get("context")
     events_result = results.get("events")
@@ -4225,11 +4273,21 @@ def _merge_live_snapshot(previous: dict | None, refreshed: dict) -> dict:
     merged["sections"] = sections
     previous_latency = previous.get("latency_ms")
     refreshed_latency = refreshed.get("latency_ms")
-    if isinstance(previous_latency, dict) or isinstance(refreshed_latency, dict):
-        merged["latency_ms"] = {
-            **(previous_latency if isinstance(previous_latency, dict) else {}),
-            **(refreshed_latency if isinstance(refreshed_latency, dict) else {}),
-        }
+    refreshed_sections = refreshed.get("sections") or {}
+    merged_latency = {
+        **(previous_latency if isinstance(previous_latency, dict) else {})
+    }
+    if isinstance(refreshed_latency, dict):
+        for section, value in refreshed_latency.items():
+            # A latency from a section that did not become fresh is a failed-
+            # attempt duration and must never overwrite a prior successful
+            # measurement. Only fresh sections may advance the latency.
+            if refreshed_sections.get(section) == "fresh" and _valid_latency_ms(value):
+                merged_latency[section] = value
+            elif section not in merged_latency:
+                merged_latency[section] = value
+    if merged_latency:
+        merged["latency_ms"] = merged_latency
     return merged
 
 
@@ -4367,13 +4425,13 @@ def _live_snapshot_payload(
     }
     latency = value.get("latency_ms")
     if isinstance(latency, dict) and all(
-        isinstance(item, int) and not isinstance(item, bool)
+        isinstance(item, int) and not isinstance(item, bool) and _valid_latency_ms(item)
         for item in latency.values()
     ):
         payload["freshness"]["latency_ms"] = {
             section: item
             for section, item in latency.items()
-            if section in LIVE_SNAPSHOT_SECTIONS
+            if section in LIVE_SNAPSHOT_SECTIONS and _valid_latency_ms(item)
         }
     revision_tokens = {
         section: f"{_live_revision_generation}.{revision}"
diff --git a/tests/test_live_data_status.py b/tests/test_live_data_status.py
index 7bfc1a8..e0dbb66 100644
--- a/tests/test_live_data_status.py
+++ b/tests/test_live_data_status.py
@@ -17,24 +17,37 @@ def run_node(script: str) -> dict:
     return json.loads(result.stdout)
 
 
+# Shared JS for a mock panel that supports querySelector + appendChild,
+# exercising the real runtime DOM transitions of renderBucketLatency.
+_MAKE_PANEL = """
+function makePanel() {
+  const heading = {localName:'h3', id:'bucket-latency-heading'};
+  const rows = {innerHTML:'', hidden:false, appendChild(child) { this.innerHTML += (child.outerHTML || String(child)); return child; }, querySelectorAll() { return []; }};
+  const panel = {hidden:false, querySelector(sel) { if (sel === '#bucket-latency-heading') return heading; if (sel === '#bucket-latency-rows') return rows; return null; }};
+  return {panel, rows, heading};
+}
+"""
+
+_REQUIRE = "const status = require(" + json.dumps(str(STATUS)) + ");"
+
+
 def test_live_data_status_summarizes_each_feed_without_claiming_live():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-const healthy = {{fresh_for_seconds:8, sections:{{
-  context:{{age_seconds:1}}, notifications:{{age_seconds:2}}, events:{{age_seconds:3}}
-}}}};
-const oneDelayed = {{fresh_for_seconds:8, retry_in_seconds:30, sections:{{
-  context:{{age_seconds:1}}, notifications:{{age_seconds:14, stale:true, retry_in_seconds:30}}, events:{{age_seconds:3}}
-}}}};
-const twoDelayed = {{fresh_for_seconds:8, sections:{{
-  context:{{age_seconds:12, degraded:true}}, notifications:{{age_seconds:14, stale:true}}, events:{{age_seconds:3}}
-}}}};
-process.stdout.write(JSON.stringify({{
+    script = _REQUIRE + """
+const healthy = {fresh_for_seconds:8, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:2}, events:{age_seconds:3}
+}};
+const oneDelayed = {fresh_for_seconds:8, retry_in_seconds:30, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:14, stale:true, retry_in_seconds:30}, events:{age_seconds:3}
+}};
+const twoDelayed = {fresh_for_seconds:8, sections:{
+  context:{age_seconds:12, degraded:true}, notifications:{age_seconds:14, stale:true}, events:{age_seconds:3}
+}};
+process.stdout.write(JSON.stringify({
   healthy:status.describe(healthy),
   one:status.describe(oneDelayed),
   two:status.describe(twoDelayed),
-  unavailable:status.describe({{}}),
-}}));
+  unavailable:status.describe({}),
+}));
 """
     result = run_node(script)
 
@@ -51,26 +64,25 @@ process.stdout.write(JSON.stringify({{
 
 
 def test_live_data_status_controller_is_single_flight_and_reports_result():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
+    script = _REQUIRE + """
 let resolveRefresh;
 let calls = 0;
 const states = [];
-const button = {{disabled:false}};
-const output = {{textContent:''}};
-const controller = status.createRefreshController({{
+const button = {disabled:false};
+const output = {textContent:''};
+const controller = status.createRefreshController({
   button, output,
-  refresh:() => {{ calls += 1; return new Promise(resolve => {{ resolveRefresh = resolve; }}); }},
+  refresh:() => { calls += 1; return new Promise(resolve => { resolveRefresh = resolve; }); },
   onState:value => states.push(value),
-}});
-(async () => {{
+});
+(async () => {
   const first = controller.run();
   const second = controller.run();
-  const pending = {{calls, disabled:button.disabled, text:output.textContent}};
-  resolveRefresh({{context:{{}}}});
+  const pending = {calls, disabled:button.disabled, text:output.textContent};
+  resolveRefresh({context:{}});
   await Promise.all([first, second]);
-  process.stdout.write(JSON.stringify({{pending, calls, disabled:button.disabled, text:output.textContent, states}}));
-}})();
+  process.stdout.write(JSON.stringify({pending, calls, disabled:button.disabled, text:output.textContent, states}));
+})();
 """
 
     assert run_node(script) == {
@@ -83,16 +95,15 @@ const controller = status.createRefreshController({{
 
 
 def test_live_data_status_sheet_pauses_today_contains_focus_and_closes_through_back():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-function element(name) {{
-  return {{name, hidden:false, inert:false, listeners:{{}}, focused:0,
-    addEventListener(type, fn) {{ this.listeners[type] = fn; }},
-    focus() {{ this.focused += 1; }},
-  }};
-}}
+    script = _REQUIRE + """
+function element(name) {
+  return {name, hidden:false, inert:false, listeners:{}, focused:0,
+    addEventListener(type, fn) { this.listeners[type] = fn; },
+    focus() { this.focused += 1; },
+  };
+}
 const trigger = element('trigger');
-trigger.attrs = {{}};
+trigger.attrs = {};
 trigger.setAttribute = (name, value) => trigger.attrs[name] = value;
 const close = element('close');
 const refresh = element('refresh');
@@ -102,37 +113,37 @@ sheet.hidden = true;
 sheet.querySelectorAll = () => [close, refresh, back];
 const header = element('header');
 const main = element('main');
-const listeners = {{}};
-const history = {{state:null, pushes:0, backs:0,
-  pushState(state) {{ this.state=state; this.pushes += 1; }},
-  back() {{ this.backs += 1; }},
-}};
-const timerView = {{begins:[], finishes:0,
-  beginDetour(reason) {{ this.begins.push(reason); return {{identity:'issue:r:42', reason}}; }},
-  finishDetour() {{ this.finishes += 1; return {{resumed:true}}; }},
-}};
+const listeners = {};
+const history = {state:null, pushes:0, backs:0,
+  pushState(state) { this.state=state; this.pushes += 1; },
+  back() { this.backs += 1; },
+};
+const timerView = {begins:[], finishes:0,
+  beginDetour(reason) { this.begins.push(reason); return {identity:'issue:r:42', reason}; },
+  finishDetour() { this.finishes += 1; return {resumed:true}; },
+};
 const paused = element('paused');
 paused.hidden = true;
-const controller = status.createSheetController({{
+const controller = status.createSheetController({
   sheet, trigger, closeButton:close, returnButton:back, pausedStatus:paused,
-  timerView, history, historyTarget:{{addEventListener:(name, fn) => listeners[name]=fn}},
-  escapeTarget:{{addEventListener:(name, fn) => listeners[name]=fn}},
+  timerView, history, historyTarget:{addEventListener:(name, fn) => listeners[name]=fn},
+  escapeTarget:{addEventListener:(name, fn) => listeners[name]=fn},
   backgroundElements:[header, main],
-}});
+});
 controller.start();
 controller.open();
-const opened = {{hidden:sheet.hidden, expanded:trigger.attrs['aria-expanded'], paused:paused.hidden, inert:[header.inert, main.inert],
-  begins:timerView.begins, pushes:history.pushes, closeFocused:close.focused}};
+const opened = {hidden:sheet.hidden, expanded:trigger.attrs['aria-expanded'], paused:paused.hidden, inert:[header.inert, main.inert],
+  begins:timerView.begins, pushes:history.pushes, closeFocused:close.focused};
 let prevented = 0;
-listeners.keydown({{key:'Tab', target:back, shiftKey:false, preventDefault:()=>prevented++}});
-const trapped = {{prevented, closeFocused:close.focused}};
+listeners.keydown({key:'Tab', target:back, shiftKey:false, preventDefault:()=>prevented++});
+const trapped = {prevented, closeFocused:close.focused};
 controller.close();
-const requested = {{backs:history.backs, finishes:timerView.finishes}};
+const requested = {backs:history.backs, finishes:timerView.finishes};
 history.state = null;
-listeners.popstate({{state:null}});
-process.stdout.write(JSON.stringify({{opened, trapped, requested, closed:{{hidden:sheet.hidden, paused:paused.hidden,
+listeners.popstate({state:null});
+process.stdout.write(JSON.stringify({opened, trapped, requested, closed:{hidden:sheet.hidden, paused:paused.hidden,
   expanded:trigger.attrs['aria-expanded'], inert:[header.inert, main.inert], finishes:timerView.finishes,
-  triggerFocused:trigger.focused}}}}));
+  triggerFocused:trigger.focused}}));
 """
     result = run_node(script)
 
@@ -160,22 +171,21 @@ process.stdout.write(JSON.stringify({{opened, trapped, requested, closed:{{hidde
 
 
 def test_live_data_status_without_running_today_hides_return_and_does_not_resume():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-function element(hidden=false) {{ return {{hidden, inert:false, listeners:{{}}, attrs:{{}},
-  addEventListener(name, fn) {{ this.listeners[name]=fn; }}, focus() {{}},
-  setAttribute(name, value) {{ this.attrs[name]=value; }}, querySelectorAll() {{ return []; }} }}; }}
+    script = _REQUIRE + """
+function element(hidden=false) { return {hidden, inert:false, listeners:{}, attrs:{},
+  addEventListener(name, fn) { this.listeners[name]=fn; }, focus() {},
+  setAttribute(name, value) { this.attrs[name]=value; }, querySelectorAll() { return []; } }; }
 const sheet=element(true), trigger=element(), close=element(), returnButton=element();
 let finishes=0;
-const controller=status.createSheetController({{
+const controller=status.createSheetController({
   sheet,trigger,closeButton:close,returnButton,
-  timerView:{{beginDetour:()=>null,finishDetour:()=>finishes++}},
-}});
+  timerView:{beginDetour:()=>null,finishDetour:()=>finishes++},
+});
 controller.start();
 controller.open();
-const opened={{returnHidden:returnButton.hidden, expanded:trigger.attrs['aria-expanded']}};
+const opened={returnHidden:returnButton.hidden, expanded:trigger.attrs['aria-expanded']};
 controller.close();
-process.stdout.write(JSON.stringify({{opened, finishes}}));
+process.stdout.write(JSON.stringify({opened, finishes}));
 """
     assert run_node(script) == {
         "opened": {"returnHidden": True, "expanded": "true"},
@@ -184,22 +194,21 @@ process.stdout.write(JSON.stringify({{opened, finishes}}));
 
 
 def test_live_data_status_describe_exposes_bounded_bucket_latency():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-const measured = {{fresh_for_seconds:8, latency_ms:{{context:120, notifications:45}}, sections:{{
-  context:{{age_seconds:1}}, notifications:{{age_seconds:14, stale:true, retry_in_seconds:30}}, events:{{age_seconds:3}}
-}}}};
-const unbounded = {{fresh_for_seconds:8, latency_ms:{{context:-5, events:9000000}}, sections:{{
-  context:{{age_seconds:1}}, events:{{age_seconds:2}}, notifications:{{age_seconds:3}}
-}}}};
-const missing = {{fresh_for_seconds:8, sections:{{
-  context:{{age_seconds:1}}, events:{{age_seconds:2}}, notifications:{{age_seconds:3}}
-}}}};
-process.stdout.write(JSON.stringify({{
+    script = _REQUIRE + """
+const measured = {fresh_for_seconds:8, latency_ms:{context:120, notifications:45}, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:14, stale:true, retry_in_seconds:30}, events:{age_seconds:3}
+}};
+const unbounded = {fresh_for_seconds:8, latency_ms:{context:-5, events:9000000}, sections:{
+  context:{age_seconds:1}, events:{age_seconds:2}, notifications:{age_seconds:3}
+}};
+const missing = {fresh_for_seconds:8, sections:{
+  context:{age_seconds:1}, events:{age_seconds:2}, notifications:{age_seconds:3}
+}};
+process.stdout.write(JSON.stringify({
   measured:status.describe(measured),
   unbounded:status.describe(unbounded),
   missing:status.describe(missing),
-}}));
+}));
 """
     result = run_node(script)
 
@@ -207,24 +216,21 @@ process.stdout.write(JSON.stringify({{
     assert [(feed["key"], feed["latencyMs"]) for feed in result["measured"]["feeds"]] == [
         ("context", 120), ("notifications", 45), ("events", None),
     ]
+    # Strict telemetry: negatives and huge values are rejected, never clipped.
     assert [(feed["key"], feed["latencyMs"]) for feed in result["unbounded"]["feeds"]] == [
-        ("context", None), ("notifications", None), ("events", 3600000),
+        ("context", None), ("notifications", None), ("events", None),
     ]
     assert all(feed["latencyMs"] is None for feed in result["missing"]["feeds"])
 
 
 def test_live_data_status_renders_bucket_latency_panel_rows():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-const description = status.describe({{fresh_for_seconds:8, latency_ms:{{context:120, notifications:45, events:3000}}, sections:{{
-  context:{{age_seconds:1}}, notifications:{{age_seconds:14, stale:true}}, events:{{age_seconds:3}}
-}}}});
-function element() {{
-  return {{innerHTML:'', hidden:false, textContent:''}};
-}}
-const panel = element();
-const rows = status.renderBucketLatency(description, panel);
-process.stdout.write(JSON.stringify({{rows, panelHidden:panel.hidden, html:panel.innerHTML}}));
+    script = _REQUIRE + _MAKE_PANEL + """
+const description = status.describe({fresh_for_seconds:8, latency_ms:{context:120, notifications:45, events:3000}, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:14, stale:true}, events:{age_seconds:3}
+}});
+const {panel, rows, heading} = makePanel();
+const rowsReturned = status.renderBucketLatency(description, panel);
+process.stdout.write(JSON.stringify({rows:rowsReturned, panelHidden:panel.hidden, html:rows.innerHTML, headingId:heading.id}));
 """
     result = run_node(script)
 
@@ -232,7 +238,10 @@ process.stdout.write(JSON.stringify({{rows, panelHidden:panel.hidden, html:panel
     assert result["rows"][0] == {"key": "context", "label": "Work", "latencyMs": 120, "stale": False}
     assert result["rows"][2]["latencyMs"] == 3000
     assert result["panelHidden"] is False
+    assert result["headingId"] == 'bucket-latency-heading'
     assert 'data-bucket="context"' in result["html"]
+    assert 'role="listitem"' in result["html"]
+    assert result["html"].count('role="listitem"') == 3
     assert "120 ms" in result["html"]
     assert "3.0 s" in result["html"]
     # A stale feed's number is a last-known measurement and must say so.
@@ -241,43 +250,38 @@ process.stdout.write(JSON.stringify({{rows, panelHidden:panel.hidden, html:panel
 
 
 def test_live_data_status_marks_unmeasured_buckets_honestly():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-const description = status.describe({{fresh_for_seconds:8, latency_ms:{{context:120}}, sections:{{
-  context:{{age_seconds:1}}, notifications:{{age_seconds:2}}, events:{{age_seconds:3}}
-}}}});
-function element() {{
-  return {{innerHTML:'', hidden:false, textContent:''}};
-}}
-const panel = element();
-const rows = status.renderBucketLatency(description, panel);
-process.stdout.write(JSON.stringify({{rows, html:panel.innerHTML}}));
+    script = _REQUIRE + _MAKE_PANEL + """
+const description = status.describe({fresh_for_seconds:8, latency_ms:{context:120}, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:2}, events:{age_seconds:3}
+}});
+const {panel, rows, heading} = makePanel();
+const rowsReturned = status.renderBucketLatency(description, panel);
+process.stdout.write(JSON.stringify({panelHidden:panel.hidden, html:rows.innerHTML}));
 """
     result = run_node(script)
 
-    assert result["panelHidden"] is False if "panelHidden" in result else True
+    assert result["panelHidden"] is False
     assert 'data-bucket="events"' in result["html"]
     assert "not measured" in result["html"]
     assert "—" not in result["html"]
+    assert 'role="listitem"' in result["html"]
 
 
 def test_live_data_status_hides_latency_panel_without_any_measurement():
-    script = f"""
-const status = require({json.dumps(str(STATUS))});
-const description = status.describe({{fresh_for_seconds:8, sections:{{
-  context:{{age_seconds:1}}, notifications:{{age_seconds:2}}, events:{{age_seconds:3}}
-}}}});
-function element() {{
-  return {{innerHTML:'', hidden:false, textContent:''}};
-}}
-const panel = element();
-const rows = status.renderBucketLatency(description, panel);
-process.stdout.write(JSON.stringify({{rows, panelHidden:panel.hidden, html:panel.innerHTML}}));
+    script = _REQUIRE + _MAKE_PANEL + """
+const description = status.describe({fresh_for_seconds:8, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:2}, events:{age_seconds:3}
+}});
+const {panel, rows, heading} = makePanel();
+const rowsReturned = status.renderBucketLatency(description, panel);
+process.stdout.write(JSON.stringify({panelHidden:panel.hidden, html:rows.innerHTML, headingIntact:heading.id === 'bucket-latency-heading'}));
 """
     result = run_node(script)
 
     assert result["panelHidden"] is True
     assert result["html"] == ""
+    # Hiding must never damage the preserved heading / aria-labelledby target.
+    assert result["headingIntact"] is True
 
 
 def test_live_data_status_has_accessible_mobile_safe_sheet_contract():
@@ -308,6 +312,9 @@ def test_live_data_status_has_accessible_mobile_safe_sheet_contract():
     assert 'id="bucket-latency-rows" role="list"' in html
     assert "bucket-latency-row { display:flex" in css
     assert ".bucket-latency-row strong, .bucket-latency-value { overflow-wrap:anywhere" in css
+    # Rows must carry role=listitem so the list semantics hold at runtime.
+    assert 'role="listitem"' in status_source
+    assert "renderBucketLatency" in status_source
     assert "liveDataStatus.describe" in dashboard
     assert "liveDataStatus.createRefreshController" in dashboard
     assert "liveDataStatus.mount" in dashboard
@@ -316,3 +323,102 @@ def test_live_data_status_has_accessible_mobile_safe_sheet_contract():
     assert "createSheetController" in status_source
     assert "backgroundElements:[qs('header'), qs('main'), qs('#mobile-task-dock')]" in status_source
     assert "contextPoller.getState()" in dashboard
+
+
+def test_live_data_status_latency_lifecycle_no_data_measured_stale():
+    """Real DOM-style lifecycle across three states through the same panel:
+    no measurement -> hide (heading intact); measured -> rows with role=listitem;
+    stale-but-measured -> rows with 'last known'. The heading and aria-labelledby
+    target survive every transition and rows always use role=listitem."""
+    script = _REQUIRE + _MAKE_PANEL + """
+const noData = status.describe({fresh_for_seconds:8, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:2}, events:{age_seconds:3}
+}});
+const measured = status.describe({fresh_for_seconds:8, latency_ms:{context:120, notifications:45, events:0}, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:14, stale:true}, events:{age_seconds:3}
+}});
+const p1 = makePanel(); const p2 = makePanel(); const p3 = makePanel();
+status.renderBucketLatency(noData, p1.panel);
+status.renderBucketLatency(measured, p2.panel);
+// stale-but-measured: reuse the same measured description (notifications stale).
+status.renderBucketLatency(measured, p3.panel);
+process.stdout.write(JSON.stringify({
+  noData: {hidden:p1.panel.hidden, rowsHtml:p1.rows.innerHTML, headingId:p1.heading.id},
+  measured: {hidden:p2.panel.hidden, rowsHtml:p2.rows.innerHTML, listitemCount:(p2.rows.innerHTML.match(/role="listitem"/g)||[]).length},
+  staleMeasured: {hidden:p3.panel.hidden, hasLastKnown:p3.rows.innerHTML.includes('last known'), headingId:p3.heading.id},
+}));
+"""
+    result = run_node(script)
+
+    # No data: hidden, empty, heading preserved.
+    assert result["noData"]["hidden"] is True
+    assert result["noData"]["rowsHtml"] == ""
+    assert result["noData"]["headingId"] == "bucket-latency-heading"
+    # Measured: shown with 3 listitem rows, heading preserved.
+    assert result["measured"]["hidden"] is False
+    assert result["measured"]["listitemCount"] == 3
+    # Stale-but-measured: still shown with 'last known', heading preserved.
+    assert result["staleMeasured"]["hidden"] is False
+    assert result["staleMeasured"]["hasLastKnown"] is True
+    assert result["staleMeasured"]["headingId"] == "bucket-latency-heading"
+
+
+def test_live_data_status_strict_latency_telemetry_rejects_malformed_values():
+    """Client telemetry accepts only finite nonnegative bounded integers.
+    Rejects null, bool, strings, arrays, floats, negatives, Infinity, and
+    huge values without Number() coercion or clipping."""
+    script = _REQUIRE + """
+function latencyOf(value) {
+  const desc = status.describe({fresh_for_seconds:8, latency_ms:{context:value}, sections:{context:{age_seconds:1}}});
+  return desc.feeds[0].latencyMs;
+}
+process.stdout.write(JSON.stringify({
+  null: latencyOf(null),
+  boolTrue: latencyOf(true),
+  boolFalse: latencyOf(false),
+  stringNum: latencyOf("120"),
+  stringWord: latencyOf("fast"),
+  array: latencyOf([120]),
+  float: latencyOf(120.9),
+  negative: latencyOf(-5),
+  infinity: latencyOf(Infinity),
+  huge: latencyOf(99999999),
+  zero: latencyOf(0),
+  valid: latencyOf(450),
+}));
+"""
+    result = run_node(script)
+
+    assert result == {
+        "null": None,
+        "boolTrue": None,
+        "boolFalse": None,
+        "stringNum": None,
+        "stringWord": None,
+        "array": None,
+        "float": None,
+        "negative": None,
+        "infinity": None,
+        "huge": None,
+        "zero": 0,
+        "valid": 450,
+    }
+
+
+def test_live_data_status_renders_submillisecond_honestly():
+    """Sub-millisecond latency (0 ms after rounding) renders as '<1 ms',
+    never a fabricated exact '1 ms' floor and never '0 ms'."""
+    script = _REQUIRE + _MAKE_PANEL + """
+const description = status.describe({fresh_for_seconds:8, latency_ms:{context:0, notifications:0, events:0}, sections:{
+  context:{age_seconds:1}, notifications:{age_seconds:2}, events:{age_seconds:3}
+}});
+const {panel, rows} = makePanel();
+status.renderBucketLatency(description, panel);
+process.stdout.write(JSON.stringify({html:rows.innerHTML}));
+"""
+    result = run_node(script)
+
+    assert "<1 ms" in result["html"]
+    assert "1 ms" not in result["html"].replace("<1 ms", "")
+    assert "0 ms" not in result["html"]
+    assert 'role="listitem"' in result["html"]
diff --git a/tests/test_live_snapshot.py b/tests/test_live_snapshot.py
index db60807..54d5d42 100644
--- a/tests/test_live_snapshot.py
+++ b/tests/test_live_snapshot.py
@@ -943,3 +943,167 @@ async def test_live_snapshot_persists_reboot_stable_wall_timestamps(monkeypatch,
 
     assert response.status_code == 200
     assert set(main._live_snapshot_store.load().created_at.values()) == {epoch}
+
+
+@pytest.mark.anyio
+async def test_failed_upstream_attempt_never_overwrites_prior_successful_latency(monkeypatch, tmp_path):
+    """A failed upstream-attempt duration must never overwrite the previous
+    successful latency, and is never mislabeled as 'last known'. With no prior
+    success the section shows 'not measured'."""
+    epoch = 1000.0
+    monkeypatch.setattr(main, "_live_snapshot_clock", lambda: epoch)
+    monkeypatch.setattr(
+        main,
+        "_live_snapshot_store",
+        LiveSnapshotStore(tmp_path / "lat.sqlite3", clock=lambda: main._live_snapshot_clock()),
+    )
+
+    async def user():
+        return {"id": 1, "login": "timmy"}
+
+    async def empty():
+        return []
+
+    async def failing_notifications():
+        raise ConnectionError("Gitea notifications endpoint refused connection")
+
+    monkeypatch.setattr(main, "current_user", user)
+    monkeypatch.setattr(main, "repos", empty)
+    monkeypatch.setattr(main, "issues", empty)
+    monkeypatch.setattr(main, "pull_requests", empty)
+    monkeypatch.setattr(main, "activity_events", lambda _user: empty())
+
+    async def healthy_notifications():
+        return {"items": [], "page": 1, "total": 0, "has_more": False}
+
+    monkeypatch.setattr(main, "notifications", healthy_notifications)
+
+    # First call: all sections succeed, each records a real latency.
+    result = payload(await main.live_snapshot())
+    first_latency = dict(result["freshness"]["latency_ms"])
+    assert set(first_latency) == {"context", "events", "notifications"}
+    for value in first_latency.values():
+        assert isinstance(value, int) and value >= 0
+
+    # Second call: notifications fails upstream. The failed-attempt duration
+    # must NOT overwrite the prior successful notifications latency.
+    monkeypatch.setattr(main, "notifications", failing_notifications)
+
+    # Advance the clock past freshness so a real re-fetch occurs.
+    epoch += main.LIVE_SNAPSHOT_FRESHNESS_SECONDS + 1
+    await main.live_snapshot()
+    # The re-fetch runs as a shared background refresh; wait for it so the
+    # merged result (with prior latency preserved) is published.
+    assert main._live_snapshot_task is not None
+    await main._live_snapshot_task
+    result2 = payload(await main.live_snapshot())
+    second_latency = result2["freshness"]["latency_ms"]
+
+    # context/events remain measured (they succeeded again); notifications
+    # retains its PRIOR successful latency, not the failed-attempt duration.
+    assert second_latency["notifications"] == first_latency["notifications"]
+    assert second_latency["notifications"] >= 0
+    # The failed section is marked stale or unavailable, not 'last known'.
+    assert result2["sections"]["notifications"] in ("stale", "temporarily unavailable")
+    # Ensure the failed-attempt's own timing never leaked into notifications.
+    assert second_latency["events"] >= first_latency["events"]
+
+
+@pytest.mark.anyio
+async def test_submillisecond_upstream_measures_zero_no_fabricated_floor(monkeypatch):
+    """Sub-millisecond operations measure as 0, never a fabricated 1 ms floor."""
+    async def fast_user():
+        return {"id": 1, "login": "timmy"}
+
+    async def fast_empty():
+        return []
+
+    monkeypatch.setattr(main, "current_user", fast_user)
+    monkeypatch.setattr(main, "repos", fast_empty)
+    monkeypatch.setattr(main, "issues", fast_empty)
+    monkeypatch.setattr(main, "pull_requests", fast_empty)
+    monkeypatch.setattr(main, "activity_events", lambda _user: fast_empty())
+    monkeypatch.setattr(main, "notifications", fast_empty)
+
+    result = payload(await main.live_snapshot())
+    latency = result["freshness"]["latency_ms"]
+
+    for section in ("context", "events", "notifications"):
+        value = latency[section]
+        assert isinstance(value, int) and not isinstance(value, bool)
+        assert value >= 0
+        # Must never be a fabricated positive floor from sub-ms work.
+        assert value != 1 or value >= 1000  # 1 ms is only legitimate at >=1ms
+
+
+@pytest.mark.anyio
+async def test_notifications_fetch_starts_before_auth_completes(monkeypatch):
+    """Notifications is auth-independent: its fetch starts immediately on the
+    same tick as the shared current_user() fetch. An 80 ms auth request and an
+    80 ms Notifications request complete concurrently (~80 ms) rather than
+    serially (~160 ms)."""
+
+    import time
+
+    notifications_started = asyncio.Event()
+    auth_started = asyncio.Event()
+    concurrency = {"notifications_started_before_auth_slept": False}
+
+    async def slow_notifications():
+        notifications_started.set()
+        await asyncio.sleep(0.08)
+        return {"items": [], "page": 1, "total": 0, "has_more": False}
+
+    async def slow_user():
+        auth_started.set()
+        # Record whether notifications had already started while auth was
+        # still awaiting — direct proof of concurrency, not serialization.
+        if notifications_started.is_set():
+            concurrency["notifications_started_before_auth_slept"] = True
+        await asyncio.sleep(0.08)
+        if notifications_started.is_set():
+            concurrency["notifications_started_before_auth_slept"] = True
+        return {"id": 1, "login": "timmy"}
+
+    async def empty():
+        return []
+
+    monkeypatch.setattr(main, "current_user", slow_user)
+    monkeypatch.setattr(main, "repos", empty)
+    monkeypatch.setattr(main, "issues", empty)
+    monkeypatch.setattr(main, "pull_requests", empty)
+    monkeypatch.setattr(main, "activity_events", lambda _user: empty())
+    monkeypatch.setattr(main, "notifications", slow_notifications)
+
+    # Drive the snapshot builder directly so the measurement is deterministic
+    # and not inflated by store/SQLite round-trips in the HTTP layer.
+    wall_start = time.perf_counter()
+    result = await main._build_live_snapshot()
+    total = time.perf_counter() - wall_start
+
+    # Both fetches were attempted.
+    assert auth_started.is_set()
+    assert notifications_started.is_set()
+    # Notifications started while auth was still in flight (concurrent, not
+    # serialized behind the shared auth fetch).
+    assert concurrency["notifications_started_before_auth_slept"] is True
+    # If serialized, total would be ~160 ms. Concurrent means ~80 ms.
+    # The bound must sit strictly between the concurrent (~80 ms) and serial
+    # (~160 ms) costs to deterministically prove concurrency.
+    assert total < 0.11
+
+
+def test_strict_latency_validation_backend_rejects_malformed():
+    """Backend strict-type gate for latency telemetry rejects everything that
+    is not a finite nonnegative bounded integer and never coerces/clips."""
+    for value in [None, True, False, "120", "fast", [120], 120.9, -5, float("inf"), 99999999]:
+        assert main._valid_latency_ms(value) is False, value
+    assert main._valid_latency_ms(0) is True
+    assert main._valid_latency_ms(1) is True
+    assert main._valid_latency_ms(3600000) is True
+    assert main._valid_latency_ms(3600001) is False
+    assert main._measured_latency_ms(0.0) == 0
+    assert main._measured_latency_ms(0.0004) == 0  # sub-ms rounds to 0, no 1ms floor
+    assert main._measured_latency_ms(-1.0) is None
+    assert main._measured_latency_ms(float("inf")) is None
+