Here is the solution:

var tv = new qx.ui.pageview.tabview.TabView;
tv.set({ width : 'auto', height : 'auto' });
tv.getPane().setHeight('auto');   // This step is important
 
var b1 = new qx.ui.pageview.tabview.Button('One');
var b2 = new qx.ui.pageview.tabview.Button('Two');
tv.getBar().add(b1, b2);
 
var p1 = new qx.ui.pageview.tabview.Page(b1);
p1.add(new qx.ui.basic.Atom('Page one'));
 
var p2 = new qx.ui.pageview.tabview.Page(b2);
p2.add(new qx.ui.basic.Atom('Page two'));
 
tv.getPane().add(p1, p2);
tv.getBar().add(b1, b2);