One common way of moving the focus around in a user interface on Windows is to use the Tab key to move sequentially between the controls. The contents of a WPF toolbar seems to act like a kind of tabbing black-hole. The focus goes in there but never comes out again. Try pasting this XAML code into the XAMLPad to see the problem.
Fortunately this can be easily fixed by using the "KeyboardNavigation" attached property and changing the TabNavigation to ’Continue’. This allows the tab focus to move out of the toolbar and back to the other elements of the UI.