出处：http://docs.sencha.com/extjs/4.2.1/extjs-build/examples/layout-browser/layouts/combination.js
// A common mistake when adding grids to a layout is creating a panel first,
// then adding the grid to it. GridPanel (xtype:'grid') is a Panel subclass,
// so you can add it directly as an item into a container. Typically you will
// want to specify layout:'fit' on GridPanels so that they'll size along with
// their container and take up the available space.
大致意思是Grid是Panel的子类，不需要用Panel再包Grid了，这样做的话，会导致Grid的Fit布局，不能奏效了。
示例页面：http://docs.sencha.com/extjs/4.2.1/extjs-build/examples/layout-browser/layout-browser.html
选左边树上的Combination Examples下的Tabs With Nested Layouts
选Foo页的Inner Tab 2，这下边有个“下置标签选项卡”，再选择“Nested Grid”，就能Grid自适应的效果。
注意：因为Panel有closable属性，Grid是子类，当然也有啦。原来Grid也是可以“关闭”的啊！