title: OpenLayers destroyFeatures() doesn't destroy features when using a clustering strategy
body: <p>A similar issue to that described in <a href="https://stackoverflow.com/questions/11869938/openlayers-destroyed-features-reappear-after-zooming-in-or-out">OpenLayers: destroyed features reappear after zooming in or out</a></p>
  
  <p>Call destroyFeatures() or removeAllFeatures() (or both, in either order) on a vector layer. The features disappear from view. But then zoom in or out, and they reappear. For me, this only happens if a clustering strategy is being used. It almost seems as if the cluster feature is destroyed but the underlying features represented by that cluster are not, so, when you zoom in or out the clustering is recalculated from those underlying features and re-drawn.</p>
  
  <p>Googling reveals a number of conversations among the developers of OpenLayers a few years ago concerning issues with destroyFeatures(). It's surprising that, even now, the issues don't seem to have been fully resolved.</p>
  
  <p>I can get around the problem by destroying the whole layer (using destroy()) and then recreating it when needed. That's OK in my case, but I can imagine cases where such a blunderbuss approach might not be desirable.</p>
  
  <p>In response to the request for a code sample, here is an abbreviated version of the code in the version that is working (ie, using destroy()). In the non-working version, I called destroyFeatures() instead (and did not set the layer to null). As stated above, this would erase the features initially, but if I then zoomed in or out using this.map.zoomIn(), the features would reappear.</p>
  
  <p>Note 1: the functions are called from Objective-C via the JavaScript bridge.
  Note 2: the JavaScript was generated using CoffeeScript.</p>
  
  <pre><code>(function() {
    var addSightingsLayer, displayFeaturesForSightingsWithGeoJSONData, geoJSONFormat, load, map, projectionSphericalMercator, projectionWGS84, removeSightingsLayer, sightingsLayer;
  
    addSightingsLayer = function() {
      var context, layerStyle, layerStyleSelected, style, styleMap, styleSelected, yerClusteringStrategy;
      if (!(this.sightingsLayer === null)) return;
      yerClusteringStrategy = new OpenLayers.Strategy.Cluster({
        distance: 10,
        threshold: 2
      });
      this.sightingsLayer = new OpenLayers.Layer.Vector('Sightings', {
        strategies: [yerClusteringStrategy]
      });
      this.map.addLayer(this.sightingsLayer);
      style = {
        // Here I define a style
      };
      context = {
      // Here I define a context, with several functions depending on whether there is a cluster or not, eg:
        dependentLabel: function(feature) {
          if (feature.cluster) {
            return feature.attributes.count;
          } else {
            return feature.attributes.name;
          }
        }, ....
  
      };
      layerStyle = new OpenLayers.Style(style, {
        context: context
      });
      styleSelected = {
        //...
      };
      layerStyleSelected = new OpenLayers.Style(styleSelected, {
        context: context
      });
      styleMap = new OpenLayers.StyleMap({
        'default': layerStyle,
        'select': layerStyleSelected
      });
      this.sightingsLayer.styleMap = styleMap;
    };
  
    removeSightingsLayer = function() {
      if (this.sightingsLayer === null) return;
      this.sightingsLayer.destroy();
      return this.sightingsLayer = null;
    };
  
    displayFeaturesForSightingsWithGeoJSONData = function(geoJSONData) {
      if (this.sightingsLayer === null) JFOLMap.addSightingsLayer();
      return this.sightingsLayer.addFeatures(this.geoJSONFormat.read(geoJSONData));
    };
  
    load = function() {
      var lat, lon, osmLayer, zoom;
      lat = ...;
      lon = ...;
      zoom = ...;
      this.map = new OpenLayers.Map('mapDiv', {
        controls: ...,
        eventListeners: ...
      });
      osmLayer = new OpenLayers.Layer.OSM();
      this.map.addLayer(osmLayer);
      return this.map.setCenter(new OpenLayers.LonLat(lon,     lat).transformWGS84ToSphericalMercator(), zoom);
    };
  
    OpenLayers.LonLat.prototype.transformWGS84ToSphericalMercator = function() {
      return this.transform(JFOLMap.projectionWGS84, JFOLMap.projectionSphericalMercator);
    };
  
    OpenLayers.LonLat.prototype.transformSphericalMercatorToWGS84 = function() {
      return this.transform(JFOLMap.projectionSphericalMercator, JFOLMap.projectionWGS84);
    };
  
    map = null;
    sightingsLayer = null;
    sightingsPopoverControl = null;
    projectionWGS84 = new OpenLayers.Projection('EPSG:4326');
    projectionSphericalMercator = new OpenLayers.Projection('EPSG:900913');
    geoJSONFormat = new OpenLayers.Format.GeoJSON({
      'externalProjection': projectionWGS84,
      'internalProjection': projectionSphericalMercator
    });
  
    this.JFOLMap = {
      map: map,
      sightingsLayer: sightingsLayer,
      projectionSphericalMercator: projectionSphericalMercator,
      projectionWGS84: projectionWGS84,
      geoJSONFormat: geoJSONFormat,
      load: load,
      addSightingsLayer: addSightingsLayer,
      removeSightingsLayer: removeSightingsLayer,
      displayFeaturesForSightingsWithGeoJSONData: displayFeaturesForSightingsWithGeoJSONData,
    };
  
  }).call(this);
  </code></pre>