By default, Microsoft Outlook allows you to synchronize the Navigation Pane width in expanded mode with its Popup Window width.
This example illustrates how to add similar behavior to the NavBar.
To add this functionality, it is necessary to add event handlers for NavPaneExpandedChanged and NavPaneExpandedChanging events, which will keep the NavBar and Popup actual widths synchronized when they are changed.
Then it is necessary to create a NavigationPaneView descendant and override the UpdatePresenterTemplateByEnabledState method, which should set your stored width for NavBar before the latter changes its visual state.
To learn more on how to implement similar functionality in Silverlight, refer to the T246669 example.
