findStaleTile_(tileCoord, tilesByZ) {
    const tileCache = this.tileCache_;
    const z = tileCoord[0];
    const x = tileCoord[1];
    const y = tileCoord[2];
    const staleKeys = this.getStaleKeys();
    for (let i = 0; i < staleKeys.length; ++i) {
      const cacheKey = getCacheKey(
        this.getLayer().getSource(),
        staleKeys[i],
        z,
        x,
        y,
      );
      if (tileCache.containsKey(cacheKey)) {
        const tile = tileCache.peek(cacheKey);
        if (tile.getState() === TileState.LOADED) {
          tile.endTransition(getUid(this));
          addTileToLookup(tilesByZ, tile, z);
          return true;
        }
      }
    }
    return false;
  }
Look for tiles covering the provided tile coordinate at an alternate
zoom level.  Loaded tiles will be added to the provided tile texture lookup.
@param {import("../../tilecoord.js").TileCoord} tileCoord The target tile coordinate.
@param {TileLookup} tilesByZ Lookup of tiles by zoom level.
@return {boolean} The tile coordinate is covered by loaded tiles at the alternate zoom level.
@private