TMS tiles render upside down because the Y axis counts from the bottom

• At z18 Beijing is stitched into the southern hemisphere and the Taihang range runs inverted, while left-right orientation stays correct
• The Beijing tile is y=105000 in XYZ but y=157143 in the TMS directory, and the two values sum to 262143
• The same tile set displays correctly in QGIS when opened as a TMS layer

1. Open one tile of known location and back-compute its lat/lng to see whether north and south are swapped
2. Read the top row's y value: close to 2^z indicates TMS numbering
3. Check the service documentation; GeoServer GWC exposes both `/gwc/service/tms/1.0.0` and XYZ
4. Request `y=0` and `y=2^z-1` manually and see which one is the south pole
5. Apply `y_tms = 2^z - 1 - y_xyz` and reload

Use Leaflet's `{-y}` placeholder (`L.tileLayer('/tiles/{z}/{x}/{-y}.png')`), or in OpenLayers flip inside `tileUrlFunction`: `const n = Math.pow(2, z); return '/tiles/' + z + '/' + x + '/' + (n - 1 - y) + '.png'`.

const z = 18, n = Math.pow(2, z);     // 262144
const yXyz = 105000;
const yTms = n - 1 - yXyz;            // 157143

L.tileLayer('/tiles/{z}/{x}/{-y}.png', {
  tileSize: 256,
  minZoom: 3,
  maxZoom: 18,
  tms: false
}).addTo(map);
L.tileLayer('/tiles/{z}/{x}/{y}.png', { tileSize: 256 }).addTo(map);

• Combining `tms: true` with `{-y}`, which flips twice and restores the inverted map
• Hardcoding one level's `2^z - 1` constant instead of computing the flip per zoom