This is down the to autoMaximize method in Ext.Viewport which tries to hide the address bar when it exists.
In the next release we have decided to change the default value of this to false, instead of true. There are just too many situations where there are issues with the viewport not expanding. Not to mention the fact that the performance is slightly worst when it is enabled.
If you ever want to enable it in your application int he future, you can use the following configuration in Ext.setup or Ext.application: