Merge pull request #920 from ahocevar/backbuffer-conditional

Backbuffer and transitionend only when there are tiles. r=@bartvde

lib/OpenLayers/Layer/Grid.js

@@ -1112,10 +1112,17 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             //if that was the last tile, then trigger a 'loadend' on the layer
             if (this.numLoadingTiles === 0) {
                 if (this.backBuffer) {
-                    this._transitionElement = tile.imgDiv;
+                    if (this.backBuffer.childNodes.length === 0) {
-                    for (var i=this.transitionendEvents.length-1; i>=0; --i) {
+                        // no tiles transitioning, remove immediately
+                        this.removeBackBuffer();
+                    } else {
+                        // wait until transition has ended or delay has passed
+                        this._transitionElement = aborted ?
+                            this.div.lastChild : tile.imgDiv;
+                        var transitionendEvents = this.transitionendEvents;
+                        for (var i=transitionendEvents.length-1; i>=0; --i) {
                             OpenLayers.Event.observe(this._transitionElement,
-                            this.transitionendEvents[i],
+                                transitionendEvents[i],
                                 this._removeBackBuffer);
                         }
                         // the removal of the back buffer is delayed to prevent
@@ -1124,6 +1131,7 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
                             this._removeBackBuffer, this.removeBackBufferDelay
                         );
                     }
+                }
                 this.loading = false;
                 this.events.triggerEvent("loadend");
             }

tests/Layer/Grid.html

@@ -1334,7 +1334,9 @@
         t.ok(layer.backBuffer === map.layerContainerDiv.firstChild,
              '[a] back buffer is first child of layer container div');
 
-        // Mark one tile loaded, to see if back buffer removal gets scheduled.
+        // Mark one tile loaded and add an element to the backbuffer, to see if
+        // backbuffer removal gets scheduled.
+        layer.backBuffer.appendChild(document.createElement('img'));
         layer.grid[1][1].onImageLoad();
 
         t.ok(layer.backBufferTimerId !== null,