问题现象:
解决办法:http://extjs.com/learn/Ext_FAQ_Forms#My_Combobox_in_a_tab.2C_or_card_layout_does_not_assume_the_correct_width
注意:这里的 hideMode: 'offsets' 需要设置在Combobox 的上层控件中,如Panel或FormPanel