What does the Properties tab show?
If you have opened a diagram and display the 
Properties bar, the following properties are displayed:
You have selected elements in the diagram window and the focus is not on an element in the navigation.
If you have selected an element, its properties are displayed.
If you select two or more similar elements, the properties that the selected elements have in common are displayed.
You have not selected an element in the diagram window and the focus is not on an element in the navigation: The properties of the diagram are displayed.
If the focus is on an element in the navigation, its properties are displayed even if elements are selected in the diagram.
If the focus is on an element in the navigation and other elements are selected, the properties that the selected elements have in common are displayed. This is also the case if elements are selected in the diagram.