Tiles return 204 instead of 404, leaving invisible white gaps

• The network panel shows a run of `204 No Content` responses for tiles outside coverage
• `tileerror` never fires and the basemap failure notice never appears
• `e.tile.naturalWidth` is 0 in the `tileload` handler for those tiles

1. Filter the network panel by status 204, or by 200 responses with a 0-byte body
2. Inspect `Content-Type` and `Content-Length` on the tile response
3. Log `e.tile.naturalWidth` inside `tileload` to catch zero-size images
4. Check the tile server's documented empty-tile policy: 204, transparent PNG, or 404
5. Reproduce with a coordinate well outside coverage

Make the server return 404 for missing tiles (`try_files $uri =404`) so the client can react, or return a 1x1 transparent PNG; on the client, count `tileload` events with `naturalWidth === 0` and show a no-data hint after a few.

layer.on('tileload', e => {
  if (e.tile.naturalWidth === 0) console.warn('空瓦片', e.coords, e.tile.src);
});
// 204 的瓦片会打出警告，而 tileerror 不会触发

# Nginx：空目录不再返回 204，而是 404
location /tiles/ {
  try_files $uri =404;
}

# 若希望缺图时不破坏画面，返回 1×1 透明 PNG
location /tiles/ {
  try_files $uri /blank.png;
  add_header Cache-Control "public, max-age=86400";
}

• Monitoring only `tileerror` counts and concluding the layer is fine, because 204 is a 2xx success
• Replacing 204 with a 200 HTML error page, which some browsers report as an error event and others render as a broken image