When using a buffer, the grid lacks rows/columns at the top/left.

This is because the threshold used for deciding when a column or row is shifted is too far to the bottom right. A tiny fix, but effective. A new test makes sure that we don't shift columns more than necessary when the layer is dragged.

lib/OpenLayers/Layer/Grid.js

@@ -1193,7 +1193,7 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             );
             return;
         }
-        var buffer = this.buffer || 1;
+        var buffer = this.buffer + 1;
         var scale = this.getResolutionScale();
         while(true) {
             var tlViewPort = {

tests/Layer/Grid.html

@@ -239,6 +239,23 @@
         t.eq( zoom, 2, "getZoomForExtent() returns correct value");
     }   
     
+    function test_moveGriddedTiles(t) {
+        t.plan(1);
+        var map = new OpenLayers.Map('map');
+        layer = new OpenLayers.Layer.WMS(name, url, params, {buffer: 2});
+        map.addLayer(layer);
+        map.setCenter([0, 0], 5);
+        var count = 0;
+        layer.shiftColumn = function(prepend) {
+            ++count;
+            OpenLayers.Layer.WMS.prototype.shiftColumn.apply(this, arguments);
+        }
+        map.moveTo([15, 0]);
+        t.delay_call(.5, function() {
+            t.eq(count, 1, "column shifted once");
+        });
+    }
+    
     function test_Layer_Grid_moveTo(t) {
 
         t.plan(17);