阻止WPF TextBox随着文本的增长而停止

I am trying to modify my simple control so that the text box does not grow as the user types in long text. I took a look at some solutions posted here at Stackoverflow that suggest using a Grid and an invisible Border and binding the Width of the text box to the ActualWidth of the Border, but I can't seem to get it to work in my setup.I am trying to modify my simple control so that