Add sortable-group#prepare to stash itemPosition

itemPosition causes synchronous layout recalculations. We want
it happening as little as possible. Now it is stashed on
_startDrag and cleared on complete.

addon/components/sortable-group.js

@@ -56,14 +56,30 @@ export default Component.extend({
     this.get('items').removeObject(item);
   },
 
+  /**
+    Prepare for sorting.
+    Main purpose is to stash the current itemPosition so
+    we don’t incur expensive re-layouts.
+
+    @method prepare
+  */
+  prepare() {
+    this._itemPosition = this.get('itemPosition');
+  },
+
   /**
     Update item positions.
 
     @method update
   */
   update() {
     let sortedItems = this.get('sortedItems');
-    let y = this.get('itemPosition');
+    let y = this._itemPosition;
+
+    // Just in case we haven’t called prepare first.
+    if (y === undefined) {
+      y = this.get('itemPosition');
+    }
 
     sortedItems.forEach(item => {
       if (!get(item, 'isDragging')) {
@@ -80,6 +96,8 @@ export default Component.extend({
     let items = this.get('sortedItems');
     let models = items.mapBy('model');
 
+    delete this._itemPosition;
+
     this.sendAction('onChange', models);
   }
 });

addon/mixins/sortable-item.js

@@ -89,16 +89,16 @@ export default Mixin.create({
     @type Number
   */
   y: computed(function(_, value) {
-    if (arguments.length === 2) {
+    if (arguments.length === 2 && value !== this._y) {
       this._y = value;
       this._scheduleApplyPosition();
     }
 
-    if (this._y !== undefined) {
-      return this._y;
-    } else {
-      return this.element.offsetTop;
+    if (this._y === undefined) {
+      this._y = this.element.offsetTop;
     }
+
+    return this._y;
   }),
 
   /**
@@ -179,6 +179,7 @@ export default Mixin.create({
       .on('mousemove touchmove', drag)
       .on('mouseup touchend', drop);
 
+    this._tellGroup('prepare');
     this.set('isDragging', true);
   },