Here is the setup:
3 tabs
Tabs 1 and 2 are using Ext.Component
Tab3 is using Ext.List

If Tab3 is loaded by the time Tab1 is being displayed, then transitions will work as expected. Clicking on Tab2 will reveal that content from the right hand side.

If Tab3 is still loading, then the loading mask will be displayed and once it has finished, Tab1 will be displayed, coming from the right. It will look fine, but if you click on Tab2, the transition will happen the wrong way around (coming from the left), as if the current tab was Tab3.

Even if the loading mask is attached to Tab3, this does sound like a bug since Tab1 is being displayed.

What do you think?