Fix scope viewer: initialize div state from data range to prevent empty CRT

xDiv/yDiv were initialized as null and excluded from the main uPlot
effect's dependency array, causing the effect to bail on its null guard
and never re-run when the init effect set them to computed values.

frontend/src/components/notebook/output/ScopeWaveformViewer.tsx

@@ -104,18 +104,8 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
     return waveform.variables.length > 1 ? waveform.variables[1].type : 'voltage';
   }, [waveform, isAC]);
 
-  // State
-  const [activeTrace, setActiveTrace] = useState(0);
-  const [xDiv, setXDiv] = useState<number | null>(null);
-  const [yDiv, setYDiv] = useState<number | null>(null);
-  const [skinId, setSkinId] = useState<SkinId>(() => {
-    const stored = safeGetItem('spicebook-scope-skin') as SkinId | null;
-    return stored && SKINS[stored] ? stored : '465';
-  });
-
-  const skin = SKINS[skinId];
-
   // Compute data range using loop-based min/max (safe for large arrays)
+  // Must be computed before useState so div initializers can reference them
   const { xMin, xMax, yMin, yMax } = useMemo(() => {
     const [xLo, xHi] = arrayMinMax(waveform.x_data);
     const yData = isAC && waveform.y_magnitude_db
@@ -125,7 +115,18 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
     return { xMin: xLo, xMax: xHi, yMin: yLo, yMax: yHi };
   }, [waveform, isAC]);
 
-  // Initialize div values from data range
+  // State — xDiv/yDiv initialized from data range (never null)
+  const [activeTrace, setActiveTrace] = useState(0);
+  const [xDiv, setXDiv] = useState(() => autoDiv(xMin, xMax, NUM_H_DIVS, xSeqForType(xType)));
+  const [yDiv, setYDiv] = useState(() => autoDiv(yMin, yMax, NUM_V_DIVS, ySeqForType(yType)));
+  const [skinId, setSkinId] = useState<SkinId>(() => {
+    const stored = safeGetItem('spicebook-scope-skin') as SkinId | null;
+    return stored && SKINS[stored] ? stored : '465';
+  });
+
+  const skin = SKINS[skinId];
+
+  // Re-initialize div values when data range changes (new waveform loaded)
   useEffect(() => {
     setXDiv(autoDiv(xMin, xMax, NUM_H_DIVS, xSeqForType(xType)));
     setYDiv(autoDiv(yMin, yMax, NUM_V_DIVS, ySeqForType(yType)));
@@ -133,7 +134,7 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
 
   // Build and mount uPlot (only on data/size changes, NOT on trace/div changes)
   useEffect(() => {
-    if (!screenRef.current || width <= 0 || xDiv === null || yDiv === null) return;
+    if (!screenRef.current || width <= 0) return;
 
     const el = screenRef.current;
     const rect = el.getBoundingClientRect();
@@ -219,7 +220,7 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
   // Update X/Y scale via setScale (no chart rebuild)
   useEffect(() => {
     const u = uplotRef.current;
-    if (!u || xDiv === null || yDiv === null) return;
+    if (!u) return;
 
     const xMid = (xMin + xMax) / 2;
     const yMid = (yMin + yMax) / 2;
@@ -240,7 +241,6 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
 
   const stepXDiv = useCallback(() => {
     setXDiv(prev => {
-      if (prev === null) return prev;
       const seq = xSeqForType(xType);
       const next = step125(prev, seq, 1);
       if (next === prev) return seq[0];
@@ -250,7 +250,6 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
 
   const stepYDiv = useCallback(() => {
     setYDiv(prev => {
-      if (prev === null) return prev;
       const seq = ySeqForType(yType);
       const next = step125(prev, seq, 1);
       if (next === prev) return seq[0];
@@ -273,8 +272,8 @@ export function ScopeWaveformViewer({ waveform, width, onExitScope }: ScopeWavef
     : 0;
 
   const currentTraceName = traceNames[activeTrace] || '\u2014';
-  const xDivLabel = xDiv !== null ? formatEng(xDiv, divUnit(xType) + '/div') : '\u2014';
+  const xDivLabel = formatEng(xDiv, divUnit(xType) + '/div');
-  const yDivLabel = yDiv !== null ? formatEng(yDiv, divUnit(yType) + '/div') : '\u2014';
+  const yDivLabel = formatEng(yDiv, divUnit(yType) + '/div');
 
   return (
     <div className="scope-skin-active">

frontend/src/styles/scope-skin.css

@@ -125,8 +125,10 @@
 
 /* uPlot chart fills the CRT screen */
 .scope-skin-active .scope-screen .uplot {
-  position: absolute;
+  position: absolute !important;
   inset: 0;
+  width: 100% !important;
+  height: 100% !important;
 }
 
 /* ── Graticule overlay (10 horizontal × 8 vertical) ──────── */