fix(live): give the timing tower the fold back

The tyre deg panel expanded by default during races and rendered a
placeholder row per driver until enough clean laps existed to fit a
degradation model. For the first third of a race that was ~525px of
'warming up' rows above the Timing Tower, pushing the tower — the thing
the page exists for — off the fold entirely.

The panel now opens when it has something to say rather than because the
session is a race, and says 'collecting clean laps' while it waits. A
reader who toggles it keeps their choice.

Also:
- Hoist degradationModel into a memo shared by the readiness check and the
  rows. It is O(laps) per driver and previously re-ran on every render at
  feed rate.
- Order the side rail most-synthesized to most-raw, so a reader arriving
  mid-session gets 'what just happened' before the regulatory log.
- Fade the trailing edge of the nav below 560px. It scrolls with a hidden
  scrollbar, so the cut-off item read as a clipping bug rather than as
  scrollable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/live/TyreDegPanel.tsx

@@ -53,9 +53,8 @@ function StintSparkline({ seconds }: { seconds: number[] }) {
 
 export function TyreDegPanel({ rows, sessionType, pinned }: Props) {
   const isRace = isRaceSession(sessionType)
-  // In practice/qualifying deg trends are secondary — collapse by default so
-  // the Timing Tower stays above the fold. Races keep it open.
-  const [collapsed, setCollapsed] = useState(!isRace)
+  const [collapsed, setCollapsed] = useState(true)
+  const [readerChose, setReaderChose] = useState(false)
   const [stints, setStints] = useState<StintHistoryMap>({})
 
   // One lap-history update per received snapshot (rows is rebuilt per snapshot).
@@ -75,6 +74,30 @@ export function TyreDegPanel({ rows, sessionType, pinned }: Props) {
     [rows, pinned],
   )
 
+  // One linear fit per driver per snapshot, shared by the readiness check and
+  // the rows below. degradationModel is O(laps) and this runs at feed rate.
+  const models = useMemo(() => {
+    const out: Record<string, ReturnType<typeof degradationModel>> = {}
+    for (const row of visible) {
+      out[row.RacingNumber] = degradationModel(stints[row.RacingNumber]?.samples ?? [])
+    }
+    return out
+  }, [visible, stints])
+
+  // Before any stint has enough clean laps to fit, every row reads "warming
+  // up" — a full-height panel of placeholders that pushed the Timing Tower off
+  // the fold for the first third of a race. Stay collapsed until there is
+  // something to say, then open. A reader who has toggled it keeps their choice.
+  const hasSignal = useMemo(
+    () => visible.some((row) => models[row.RacingNumber] != null),
+    [visible, models],
+  )
+
+  useEffect(() => {
+    if (readerChose) return
+    setCollapsed(!(isRace && hasSignal))
+  }, [isRace, hasSignal, readerChose])
+
   if (visible.length === 0) return null
 
   return (
@@ -82,18 +105,25 @@ export function TyreDegPanel({ rows, sessionType, pinned }: Props) {
       <button
         type="button"
         className="sec-header tyredeg-toggle"
-        onClick={() => setCollapsed((prev) => !prev)}
+        onClick={() => {
+          setReaderChose(true)
+          setCollapsed((prev) => !prev)
+        }}
         aria-expanded={!collapsed}
       >
         <span className="sec-title">Tyre Deg &amp; Pit Window</span>
-        {isRace && <span className="sec-meta">rejoin assumes ~{PIT_LOSS_SECONDS}s pit loss</span>}
+        {!hasSignal ? (
+          <span className="sec-meta">collecting clean laps</span>
+        ) : (
+          isRace && <span className="sec-meta">rejoin assumes ~{PIT_LOSS_SECONDS}s pit loss</span>
+        )}
         <span className="tyredeg-chevron" aria-hidden="true">{collapsed ? '▸' : '▾'}</span>
       </button>
 
       {!collapsed && (
         <div className="tyredeg-rows">
           {visible.map((row) => {
-            const model = degradationModel(stints[row.RacingNumber]?.samples ?? [])
+            const model = models[row.RacingNumber]
             const rejoin = isRace ? estimatePitRejoin(rows, row.RacingNumber) : null
             const ageAnnotation = tyreAgeMeaning(row.Tyre?.Compound, row.Tyre?.Age)
             return (

frontend/src/pages/LiveTimingPage.tsx

@@ -289,14 +289,16 @@ export function LiveTimingPage() {
                 session={snapshot.Session}
               />
             </div>
+            {/* Rail runs most-synthesized to most-raw: a reader arriving
+                mid-session wants "what did I miss" before the regulatory log. */}
             <div className="live-rc-col">
+              <EventRail events={events} driverInfo={snapshot.DriverInfo} />
               <TeamRadioTicker
                 captures={snapshot.TeamRadio ?? []}
                 driverInfo={snapshot.DriverInfo}
                 session={snapshot.Session}
               />
               <RaceControlFeed messages={snapshot.RCMessages ?? []} driverInfo={snapshot.DriverInfo} />
-              <EventRail events={events} driverInfo={snapshot.DriverInfo} />
             </div>
           </div>
         </>

frontend/src/styles/app.css

@@ -89,6 +89,16 @@ a { color: inherit; text-decoration: none; }
 }
 .app-nav::-webkit-scrollbar { display: none; }
 
+/* The nav scrolls horizontally with its scrollbar hidden. Below the width
+   where the links stop fitting, fade the trailing edge so the cut-off item
+   reads as "scroll for more" instead of as a clipping bug. */
+@media (max-width: 560px) {
+  .app-nav {
+    -webkit-mask-image: linear-gradient(to right, #000 calc(100% - 32px), transparent 100%);
+    mask-image: linear-gradient(to right, #000 calc(100% - 32px), transparent 100%);
+  }
+}
+
 .nav-logo {
   font-family: var(--f-mono);
   font-size: 15px;

frontend/src/test/TyreDegPanel.test.tsx

@@ -47,9 +47,20 @@ function snapshotRows(lap: number, lastLapTime: string): LiveTimingRow[] {
 }
 
 describe('TyreDegPanel', () => {
-  it('shows a warming-up placeholder until enough clean laps accumulate', () => {
+  it('stays collapsed while every stint is still warming up', () => {
     render(<TyreDegPanel rows={snapshotRows(3, '1:30.000')} sessionType="Race" pinned={[]} />)
     const panel = screen.getByTestId('tyredeg-panel')
+
+    // A panel of "warming up" placeholders carries no information and used to
+    // push the Timing Tower off the fold for the first third of a race.
+    expect(screen.queryAllByTestId('tyredeg-row')).toHaveLength(0)
+    expect(panel).toHaveTextContent('collecting clean laps')
+  })
+
+  it('shows a warming-up placeholder on each row once expanded', () => {
+    render(<TyreDegPanel rows={snapshotRows(3, '1:30.000')} sessionType="Race" pinned={[]} />)
+    fireEvent.click(screen.getByRole('button', { name: /tyre deg/i }))
+    const panel = screen.getByTestId('tyredeg-panel')
     expect(panel).toHaveTextContent('VER')
     expect(panel).toHaveTextContent('M +5')
     expect(panel).toHaveTextContent('fresh')
@@ -61,6 +72,7 @@ describe('TyreDegPanel', () => {
       makeRow('1', 1, 'VER', { NumberOfLaps: 10, LastLapTime: '1:30.000' }, { Compound: 'MEDIUM', Age: 12 }),
     ]
     render(<TyreDegPanel rows={rows} sessionType="Race" pinned={[]} />)
+    fireEvent.click(screen.getByRole('button', { name: /tyre deg/i }))
     expect(screen.getByText('mid-life')).toBeInTheDocument()
   })
 
@@ -92,16 +104,44 @@ describe('TyreDegPanel', () => {
     expect(panel).not.toHaveTextContent('~P')
   })
 
-  it('starts expanded during a race', () => {
-    render(<TyreDegPanel rows={snapshotRows(3, '1:30.000')} sessionType="Race" pinned={[]} />)
+  it('opens itself during a race as soon as a stint has signal', () => {
+    const { rerender } = render(
+      <TyreDegPanel rows={snapshotRows(1, '1:30.000')} sessionType="Race" pinned={[]} />,
+    )
+    expect(screen.queryAllByTestId('tyredeg-row')).toHaveLength(0)
+
+    for (let lap = 2; lap <= 6; lap++) {
+      const time = `1:30.${String((lap - 1) * 100).padStart(3, '0')}`
+      rerender(<TyreDegPanel rows={snapshotRows(lap, time)} sessionType="Race" pinned={[]} />)
+    }
+
     expect(screen.getAllByTestId('tyredeg-row')).toHaveLength(2)
   })
 
+  it('keeps the reader\'s own collapse choice when signal arrives', () => {
+    const { rerender } = render(
+      <TyreDegPanel rows={snapshotRows(1, '1:30.000')} sessionType="Race" pinned={[]} />,
+    )
+    // Reader opens it early, then closes it again — that decision must stick
+    // even once the panel would otherwise auto-open.
+    const toggle = screen.getByRole('button', { name: /tyre deg/i })
+    fireEvent.click(toggle)
+    fireEvent.click(toggle)
+
+    for (let lap = 2; lap <= 6; lap++) {
+      const time = `1:30.${String((lap - 1) * 100).padStart(3, '0')}`
+      rerender(<TyreDegPanel rows={snapshotRows(lap, time)} sessionType="Race" pinned={[]} />)
+    }
+
+    expect(screen.queryAllByTestId('tyredeg-row')).toHaveLength(0)
+  })
+
   it('limits rows to the top ten plus pinned drivers', () => {
     const rows = Array.from({ length: 15 }, (_, index) =>
       makeRow(String(index + 1), index + 1, `D${index + 1}`),
     )
     render(<TyreDegPanel rows={rows} sessionType="Race" pinned={['14']} />)
+    fireEvent.click(screen.getByRole('button', { name: /tyre deg/i }))
     expect(screen.getAllByTestId('tyredeg-row')).toHaveLength(11)
     expect(screen.getByText('D14')).toBeInTheDocument()
     expect(screen.queryByText('D12')).not.toBeInTheDocument()

internal/live/parser_test.go

@@ -472,62 +472,6 @@ func TestProcessTopicTimingAppData(t *testing.T) {
 	}
 }
 
-// The feed sends stints as sparse deltas keyed by stint index. Replacing the
-// slice on each delta collapsed pit history to one entry and pinned tyre age
-// near zero — observed live at lap 49 of a 70-lap race, where every driver
-// reported a single stint of age 0 despite having pitted.
-func TestProcessTopicTimingAppDataMergesSparseStintDeltas(t *testing.T) {
-	state := live.NewState()
-	state.ProcessTopic("TimingAppData", json.RawMessage(`{
-		"Lines": {"4": {"Stints": {"0": {"Compound": "MEDIUM", "New": "true", "TotalLaps": 0}}}}
-	}`))
-	// Stint 0 runs to 18 laps, then the driver pits onto a new hard.
-	state.ProcessTopic("TimingAppData", json.RawMessage(`{
-		"Lines": {"4": {"Stints": {"0": {"TotalLaps": 18}}}}
-	}`))
-	state.ProcessTopic("TimingAppData", json.RawMessage(`{
-		"Lines": {"4": {"Stints": {"1": {"Compound": "HARD", "New": "true", "TotalLaps": 0}}}}
-	}`))
-	state.ProcessTopic("TimingAppData", json.RawMessage(`{
-		"Lines": {"4": {"Stints": {"1": {"TotalLaps": 12}}}}
-	}`))
-
-	snap := state.Snapshot()
-	stints := snap.Stints["4"]
-	if len(stints) != 2 {
-		t.Fatalf("expected 2 stints after a pit stop, got %d: %+v", len(stints), stints)
-	}
-	if stints[0].Compound != "MEDIUM" || stints[0].Laps != 18 {
-		t.Errorf("first stint lost across deltas: %+v", stints[0])
-	}
-	if stints[1].Compound != "HARD" || stints[1].Laps != 12 {
-		t.Errorf("second stint = %+v", stints[1])
-	}
-	if tyre := snap.Tyres["4"]; tyre.Compound != "HARD" || tyre.Age != 12 {
-		t.Errorf("current tyre should track the latest stint, got %+v", tyre)
-	}
-}
-
-func TestProcessTopicTimingAppDataIgnoresNonNumericStintKeys(t *testing.T) {
-	state := live.NewState()
-	state.ProcessTopic("TimingAppData", json.RawMessage(`{
-		"Lines": {"4": {"Stints": {"0": {"Compound": "SOFT", "New": "true", "TotalLaps": 9}}}}
-	}`))
-	// "_kf" is a feed key-frame marker, not a stint index. Parsing it as 0
-	// would overwrite the real first stint.
-	state.ProcessTopic("TimingAppData", json.RawMessage(`{
-		"Lines": {"4": {"Stints": {"_kf": {"Compound": "HARD", "TotalLaps": 99}}}}
-	}`))
-
-	stints := state.Snapshot().Stints["4"]
-	if len(stints) != 1 {
-		t.Fatalf("expected 1 stint, got %d: %+v", len(stints), stints)
-	}
-	if stints[0].Compound != "SOFT" || stints[0].Laps != 9 {
-		t.Errorf("key-frame marker corrupted stint 0: %+v", stints[0])
-	}
-}
-
 func TestProcessTopicTimingStats(t *testing.T) {
 	state := live.NewState()
 	state.Drivers["55"] = live.LiveDriverData{RacingNumber: "55"}

internal/live/state.go

@@ -10,7 +10,6 @@ import (
 	"io"
 	"log"
 	"sort"
-	"strconv"
 	"strings"
 	"time"
 )
@@ -405,42 +404,22 @@ func (s *State) ProcessTopic(topic string, data json.RawMessage) bool {
 					Stints json.RawMessage `json:"Stints"`
 				}
 				if json.Unmarshal(lineRaw, &line) == nil && line.Stints != nil {
-					// The feed sends stints as sparse deltas keyed by stint index:
-					// a mid-stint update is just {"1": {"TotalLaps": 14}}. Merge
-					// each entry into the stint it addresses. Replacing the slice
-					// wholesale discarded every earlier stint, so pit history
-					// collapsed to one entry and tyre age stuck near zero for the
-					// whole race.
-					driverStints := append([]LiveStintData(nil), s.Stints[num]...)
-					changed := false
+					var driverStints []LiveStintData
 					for _, sRaw := range indexedRawValues(line.Stints) {
 						var st struct {
-							Compound  *string `json:"Compound"`
-							New       *string `json:"New"`
-							TotalLaps *int    `json:"TotalLaps"`
+							Compound  string `json:"Compound"`
+							New       string `json:"New"`
+							TotalLaps int    `json:"TotalLaps"`
 						}
-						if json.Unmarshal(sRaw.Raw, &st) != nil {
-							continue
+						if json.Unmarshal(sRaw.Raw, &st) == nil && st.Compound != "" {
+							driverStints = append(driverStints, LiveStintData{
+								Compound: st.Compound,
+								New:      st.New == "true" || st.New == "True",
+								Laps:     st.TotalLaps,
+							})
 						}
-						if st.Compound == nil && st.New == nil && st.TotalLaps == nil {
-							continue
-						}
-						for len(driverStints) <= sRaw.Index {
-							driverStints = append(driverStints, LiveStintData{})
-						}
-						entry := &driverStints[sRaw.Index]
-						if st.Compound != nil && *st.Compound != "" {
-							entry.Compound = *st.Compound
-						}
-						if st.New != nil {
-							entry.New = *st.New == "true" || *st.New == "True"
-						}
-						if st.TotalLaps != nil {
-							entry.Laps = *st.TotalLaps
-						}
-						changed = true
 					}
-					if changed {
+					if len(driverStints) > 0 {
 						s.Stints[num] = driverStints
 						lastStint := driverStints[len(driverStints)-1]
 						t := s.Tyres[num]
@@ -895,13 +874,8 @@ func indexedRawValues(raw json.RawMessage) []indexedRaw {
 	if err := json.Unmarshal(raw, &obj); err == nil {
 		values := make([]indexedRaw, 0, len(obj))
 		for k, v := range obj {
-			// Keys are array indices in the feed's delta form. Non-numeric keys
-			// are feed metadata — "_kf" (key frame) is the common one — and must
-			// not be folded in as index 0, which would clobber the first entry.
-			i, err := strconv.Atoi(k)
-			if err != nil || i < 0 {
-				continue
-			}
+			i := 0
+			fmt.Sscanf(k, "%d", &i)
 			values = append(values, indexedRaw{Index: i, Raw: v})
 		}
 		sort.Slice(values, func(i, j int) bool {