No magic for loadend delay

This change reintroduces the removeBackBufferDelay, and documents exactly
what it does and when it may be useful.

lib/OpenLayers/Layer/Grid.js

@@ -189,6 +189,17 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
      */
     backBufferLonLat: null,
 
+    /**
+     * APIProperty: removeBackBufferDelay
+     * {Number} Delay in ms for removing the backbuffer when all tiles have
+     *     finished loading.  Usually a value of 0 works fine.  For slow mobile
+     *     devices, when using <singleTile> layers with big image sizes, a delay
+     *     of 40-50ms may help to avoid flickers after panning.  Default is 0.
+     *     If set, the layer's loadend event will also be delayed when a
+     *     backbuffer needs to be removed.
+     */
+    removeBackBufferDelay: 0,
+
     /**
      * APIProperty: className
      * {String} Name of the class added to the layer div. If not set in the
@@ -1098,17 +1109,15 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             });
             //if that was the last tile, then trigger a 'loadend' on the layer
             if (this.tileQueue.length === 0 && this.numLoadingTiles === 0) {
-                // give the last tile time to render before firing loadend and
+                if (this.backBuffer) {
-                // removing the backbuffer.
+                    var that = this;
-                var delay = this.tileSize.w * this.tileSize.h / 32768;
+                    window.setTimeout(function() {
-                var that = this;
+                        that.onLoadEnd();
-                window.setTimeout(function() {
-                    that.loading = false;
-                    that.events.triggerEvent("loadend");
-                    if(that.backBuffer) {
                         that.removeBackBuffer();
-                    }
+                    }, this.removeBackBufferDelay);
-                }, delay);
+                } else {
+                    this.onLoadEnd();
+                }
             }
         };
         
@@ -1125,6 +1134,15 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
         });
     },
 
+    /**
+     * Method: onLoadEnd
+     * Sets the loading flag to false and triggers the loadend event.
+     */
+    onLoadEnd: function() {
+        this.loading = false;
+        this.events.triggerEvent("loadend");
+    },
+        
     /** 
      * Method: removeTileMonitoringHooks
      * This function takes a tile as input and removes the tile hooks