preferCanvas leaves ghost trails while panning

• Old circle markers remain on screen while dragging and fade only after the pointer is released.
• At zoom 16 lines look several pixels thick with dark patches where they cross.
• The effect appears above roughly 3000 features and is invisible with a few hundred.

1. Switch preferCanvas off and confirm the ghosting disappears under SVG.
2. Drop to 500 features to see whether it is a redraw-time problem.
3. Print layer.getRenderer().options for padding, tolerance and updateWhenZooming.
4. Check whether one renderer serves both points and large polygons.

Create L.canvas({ padding: 0.5, tolerance: 5, updateWhenZooming: true }) and pass it as the layer renderer, or split point and polygon layers onto separate canvas renderers so each clears independently.

console.log(layer.getRenderer().options);
// { padding: 0.1, tolerance: 0, updateWhenZooming: false }

var canvasRenderer = L.canvas({
  padding: 0.5,        // 预绘制 50% 视口边距
  tolerance: 5,        // 点击命中的像素容差
  updateWhenZooming: true
});

var layer = L.geoJSON(data, {
  renderer: canvasRenderer,
  pointToLayer: function (feature, latlng) {
    return L.circleMarker(latlng, {
      radius: 4,
      weight: 1,
      color: '#1c7ed6',
      fillOpacity: 0.6
    });
  }
}).addTo(map);

• Setting updateWhenZooming: true with 8000 features on the default padding 0.1 makes every zoom frame slower without removing the ghost.
• Raising the circle radius to hide the artifact increases overdraw and worsens the smear.