chore： 优化

src/base/list/virtualgrouplist.js

@@ -27,7 +27,7 @@ BI.VirtualGroupList = BI.inherit(BI.Widget, {
 
     render: function () {
         var self = this, o = this.options;
-        
+
         return {
             type: "bi.vertical",
             items: [{
@@ -97,12 +97,13 @@ BI.VirtualGroupList = BI.inherit(BI.Widget, {
             this.container[self.renderedIndex === -1 ? "populate" : "addItems"](items.map(function (item, i) {
                 return o.itemFormatter(item, index + i);
             }), this);
-            var addedHeight = getElementHeight() - lastHeight;
+            var elementHeight = getElementHeight();
+            var addedHeight = elementHeight - lastHeight;
             this.tree.set(cnt, addedHeight);
             this.renderedIndex = cnt;
             cnt++;
             index += o.blockSize;
-            lastHeight = this.renderedIndex === -1 ? 0 : getElementHeight();
+            lastHeight = this.renderedIndex === -1 ? 0 : elementHeight;
         }
     },