Code:
var tab = main_tab.add({ 
     items: [{ 
          xtype: content 
     }] 
})
That's an overnest in a layoutless panel.

Read your code carefully.

You are adding to "main_tab" a Panel with no configuration except that it has one child: a component of xtype: content.

That Panel will not lay out its children!

The API documentation hammers this message into the reader!