WPF: Extending combobox to take text without overflowing the control

PROBLEM: You can get the Combobox Style and template XAML from http://msdn.microsoft.com/es-es/library/ms752094(v=vs.85).aspx

As in XAML, ControlTemplate for ComboTextBox sets to be TextBox. For some reason, when lenght of the text input is greater than the width of the textbox,