I am using a TabPanel with two tabs. If either of the tabs extends Ext.List or contains any child items, the tabs will no longer dock at the bottom. Uncommenting the one commented line will demonstrate the issue.

Code:
Ext.define('ArticleList', {
    extend: 'Ext.Panel',

    config: {
        title: 'DEFAULT',
        html: 'test',
        //items: [ {html: 'item1'}, {html: 'item2'} ]
    },
});

var recentArticleList = Ext.create('ArticleList', {iconCls: 'time', title: 'Recent'});
var toolsArticleList = Ext.create('ArticleList', {iconCls: 'settings', title: 'Tools'});

Ext.application({
    name: 'Sencha',

    launch: function () {
        this.tabPanel = Ext.create("Ext.TabPanel", {
            fullscreen: true,
            tabBarPosition: 'bottom',
        });

        this.tabPanel.add(recentArticleList);
        this.tabPanel.add(toolsArticleList);
    }
});