Latitude 89 in Web Mercator yields Infinity and NaN tile indices

• A station at latitude -89.0 disappears in Leaflet while every other point renders.
• In OpenLayers the whole layer goes blank, but source.getFeatures().length still reports the correct count.
• A custom tile script prints NaN indices and request URLs containing undefined.

1. Run L.CRS.EPSG3857.latLngToPoint(L.latLng(-89, 0), 0) and check whether y exceeds 20037508.
2. Scan the data bbox for records with abs(lat) above 85.05; usually only a handful exist.
3. Check whether the tile index code validates Math.floor output with Number.isFinite.
4. Remove the offending point and reload; a restored layer confirms the diagnosis.

Clamp latitude to plus/minus 85.0511287798 before projecting, and use a polar stereographic projection when polar regions must really be shown.

const p = L.CRS.EPSG3857.latLngToPoint(L.latLng(-89, 0), 0);
console.log(p.y);            // 3.0e7 左右，已超出 20037508
console.log(Number.isFinite(p.y));   // true，但越界

const bad = L.CRS.EPSG3857.latLngToPoint(L.latLng(-90, 0), 0);
console.log(bad.y);          // Infinity
console.log(Math.floor(bad.y / 256)); // NaN

const MAX_LAT = 85.0511287798;

function clampLat(lat) {
  return Math.max(-MAX_LAT, Math.min(MAX_LAT, lat));
}

function toMercator(lng, lat) {
  const phi = clampLat(lat) * Math.PI / 180;
  const x = lng * 20037508.34 / 180;
  let y = Math.log(Math.tan(Math.PI / 4 + phi / 2)) * 6378137;
  y = Math.max(-20037508.34, Math.min(20037508.34, y));
  return [x, y];
}
function tileIndex(lng, lat, z) {
  const [x, y] = toMercator(lng, lat);
  const n = Math.pow(2, z);
  const tx = Math.floor((x + 20037508.34) / (2 * 20037508.34) * n);
  const ty = Math.floor((20037508.34 - y) / (2 * 20037508.34) * n);
  if (!Number.isFinite(tx) || !Number.isFinite(ty)) return null;
  return [tx, ty];
}

• Clamping after projection, where y is already Infinity and no arithmetic can recover it.
• Raising maxBounds or lowering minZoom in the hope that the library will filter the point out.