Navigating Treeview with arrow keys require two keystrokes

I have a WPF application with treeview. I used the sample located at How to develop treeview with checkboxes in wpf? by @pr0gg3r. When I use the arrow keys to navigate, it needs two keystrokes before each item can be accessed as shown here Treeview Navigation Pressing the arraow key once highlights the item in blue and then pressing it again shows the focus border with dotted ractangle around the text. How can I use a single keystroke to navigate? I want to hit the arrow key once and navigate one item down or up the tree.