Zoom jumps from 17 straight to 19, skipping two levels

• `zoomend` logs 15, then 17, then 19 with each wheel step
• `map.options.zoomSnap` is `0.5` and `map.options.zoomDelta` is `2`
• Tile requests for z17 and z18 never appear and those levels stay grey
• A pinch gesture can jump from 15 to 18 at once

1. Log `map.getZoom()` inside `zoomend` to measure the step size
2. Print `map.options.zoomSnap` and `map.options.zoomDelta`
3. Search for runtime code that mutates `map.options`
4. Call `map.setZoom(18)` and see whether the map can rest at 18
5. Check `maxNativeZoom` against `maxZoom` for missing levels

Keep `zoomDelta` at 1 and never below `zoomSnap` (`zoomSnap: 0.5, zoomDelta: 1, wheelPxPerZoomLevel: 120`); when the tile set ends at 17, use `maxNativeZoom: 17, maxZoom: 19` so Leaflet upscales instead of skipping levels.

map.on('zoomend', () => console.log(map.getZoom()));

console.log(map.options.zoomSnap, map.options.zoomDelta);
// 0.5 2   <- zoomDelta 为 2，滚轮一次跨两级

const map = L.map('map', {
  center: [39.909, 116.397],
  zoom: 15,
  minZoom: 10,
  maxZoom: 19,
  zoomSnap: 0.5,      // 允许半级缩放
  zoomDelta: 1,       // 滚轮一次一级
  wheelPxPerZoomLevel: 120
});
L.tileLayer('/tiles/{z}/{x}/{y}.png', {
  minZoom: 10,
  maxNativeZoom: 17,
  maxZoom: 19
}).addTo(map);

• Raising `maxZoom` to reach higher levels when tiles stop at 17, which leaves the missing levels grey
• Setting `zoomSnap: 0` without `wheelDebounceTime`, so one wheel event triggers several zoom changes