Don't use :active pseudo-class

It's slow (500 ms recalculate style vs 1ms).

css/map.css

@@ -900,7 +900,7 @@ text.point {
         ) 9 9, auto;
 }
 
-#map:active {
+#map.panning {
     cursor: pointer; /* Opera */
     cursor: url(img/cursor-grabbing.png) 9 9, auto; /* FF */
     cursor: -webkit-image-set(

js/id/renderer/map.js

@@ -181,6 +181,12 @@ iD.Map = function(context) {
 
         transformed = true;
         supersurface.style(transformProp, transform);
+
+        if (tX || tY) {
+            context.container().selectAll('#map')
+                .classed('panning', true);
+        }
+
         queueRedraw();
 
         dispatch.move(map);
@@ -189,6 +195,8 @@ iD.Map = function(context) {
     function resetTransform() {
         if (!transformed) return false;
         supersurface.style(transformProp, '');
+        context.container().selectAll('#map')
+            .classed('panning', false);
         transformed = false;
         return true;
     }