fix: crossfade via deux img overlays — canvas jamais modifié

Problème racine : modifier l'opacité du canvas Leaflet (qui vit dans un
pane GPU-composité) via CSS causait des désynchronisations non-déterministes.

Nouvelle approche :
- Canvas : jamais touché (opacité Leaflet par défaut)
- Deux <img> overlays se croisent : prev (sortant) et next (entrant)
- Après draw(), on attend le RAF interne de Leaflet, puis on capture
  le canvas via toDataURL() dans le next img
- currentSrcRef garde l'src courante pour initialiser prev au prochain tour

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/HeatMap.tsx

@@ -33,7 +33,12 @@ export function HeatMap({ transactions }: HeatMapProps) {
   const containerRef = useRef<HTMLDivElement>(null);
   const mapRef = useRef<L.Map | null>(null);
   const heatRef = useRef<L.HeatLayer | null>(null);
-  const overlayRef = useRef<HTMLImageElement | null>(null);
+  // Two img overlays that cross-fade between each other.
+  // The canvas opacity is NEVER touched — it stays at leaflet's default.
+  const prevRef = useRef<HTMLImageElement | null>(null);
+  const nextRef = useRef<HTMLImageElement | null>(null);
+  // Src of the currently visible frame (so prev can be initialised correctly)
+  const currentSrcRef = useRef<string>('');
 
   // Initialize map once
   useEffect(() => {
@@ -65,14 +70,16 @@ export function HeatMap({ transactions }: HeatMapProps) {
     };
   }, []);
 
-  // True crossfade: old frame (overlay) fades out WHILE new frame (canvas) fades in simultaneously
+  // Crossfade: two img overlays swap roles each frame.
+  // Canvas is never hidden — we only read its pixel data via toDataURL().
   useEffect(() => {
     if (!heatRef.current || !mapRef.current) return;
 
     const canvas = (heatRef.current as unknown as { _canvas?: HTMLCanvasElement })._canvas;
-    const overlay = overlayRef.current;
+    const prev = prevRef.current;
+    const next = nextRef.current;
 
-    const update = () => {
+    const draw = () => {
       const maxAmount = Math.max(...transactions.map((t) => t.amount), 1);
       const points: L.HeatLatLngTuple[] = transactions.map((tx) => [
         tx.lat,
@@ -86,43 +93,49 @@ export function HeatMap({ transactions }: HeatMapProps) {
       }
     };
 
-    if (!canvas || !overlay) {
-      update();
+    if (!canvas || !prev || !next) {
+      draw();
       return;
     }
 
-    // 1. Capture old frame into overlay
-    try {
-      overlay.src = canvas.toDataURL();
-    } catch {
-      update();
-      return;
-    }
+    // --- Phase 1 (synchronous): set start state ---
+    // prev shows the current frame (or nothing on first run)
+    prev.src = currentSrcRef.current;
+    prev.style.transition = 'none';
+    prev.style.opacity = currentSrcRef.current ? '1' : '0';
 
-    // 2. Snap overlay to visible, cancel any running transition
-    overlay.style.transition = 'none';
-    overlay.style.opacity = '1';
+    // next is hidden and will receive the incoming frame
+    next.style.transition = 'none';
+    next.style.opacity = '0';
 
-    // 3. Hide canvas (new frame will fade in from here)
-    canvas.style.transition = 'none';
-    canvas.style.opacity = '0';
+    void prev.offsetWidth; // flush CSS so transitions start cleanly
 
-    // flush both
-    void overlay.offsetWidth;
+    // Ask leaflet to draw new data (schedules an internal RAF)
+    draw();
 
-    // 4. Draw new data onto canvas (invisible at opacity 0)
-    update();
-
-    // 5. Simultaneously: canvas fades in, overlay fades out → true crossfade
-    // Both RAF ids must be cancelled on cleanup to avoid double-transition
+    // --- Phase 2 (after leaflet redraws): capture new frame, start crossfade ---
+    // leaflet.heat schedules its own RAF inside draw() above.
+    // Our raf1 is queued *after* leaflet's RAF, so when raf1 fires,
+    // leaflet has already redrawn the canvas.
     let raf2 = 0;
     const raf1 = requestAnimationFrame(() => {
       raf2 = requestAnimationFrame(() => {
-        const DURATION = '0.55s ease-in-out';
-        canvas.style.transition = `opacity ${DURATION}`;
-        canvas.style.opacity = '1';
-        overlay.style.transition = `opacity ${DURATION}`;
-        overlay.style.opacity = '0';
+        let src: string;
+        try {
+          src = canvas.toDataURL();
+        } catch {
+          return; // map torn down
+        }
+
+        currentSrcRef.current = src;
+        next.src = src;
+        void next.offsetWidth; // ensure img is decoded before transition
+
+        const DUR = '0.55s ease-in-out';
+        prev.style.transition = `opacity ${DUR}`;
+        prev.style.opacity = '0';
+        next.style.transition = `opacity ${DUR}`;
+        next.style.opacity = '1';
       });
     });
 
@@ -132,12 +145,20 @@ export function HeatMap({ transactions }: HeatMapProps) {
   return (
     <div className="w-full h-full relative" style={{ minHeight: 0 }}>
       <div ref={containerRef} className="absolute inset-0" />
+      {/* prev: outgoing frame */}
       <img
-        ref={overlayRef}
+        ref={prevRef}
         alt=""
         className="absolute inset-0 w-full h-full pointer-events-none"
         style={{ opacity: 0, zIndex: 500 }}
       />
+      {/* next: incoming frame — sits on top of prev during crossfade */}
+      <img
+        ref={nextRef}
+        alt=""
+        className="absolute inset-0 w-full h-full pointer-events-none"
+        style={{ opacity: 0, zIndex: 501 }}
+      />
     </div>
   );
 }