LineUtil

• simplify applies Douglas-Peucker with a pixel tolerance, so higher values drop more vertices.
• clipSegment returns false when the whole segment lies outside the bounds rectangle.
• polylineCenter walks cumulative length and needs a CRS object for projection.

var latlngs = polyline.getLatLngs();

var pts = latlngs.map(function (latlng) {
  return map.latLngToLayerPoint(latlng);
});

/* 容差 1.5 像素：缩放后肉眼几乎看不出差别 */
var thin = L.LineUtil.simplify(pts, 1.5);

var back = thin.map(function (p) {
  return map.layerPointToLatLng(p);
});

console.log(pts.length, '→', thin.length);
polyline.setLatLngs(back);

var size = map.getSize();
var box = new L.Bounds(L.point(0, 0), L.point(size.x, size.y));

var a = map.latLngToLayerPoint(latlngA);
var b = map.latLngToLayerPoint(latlngB);

var clipped = L.LineUtil.clipSegment(a, b, box, false, true);

if (clipped) {
  drawLine(clipped[0], clipped[1]);
}

• Passing raw LatLng arrays silently treats degrees as pixels and yields meaningless results.
• Simplification is tied to current zoom, so storing results distorts the line at other levels.