our bar is not expanded by default. At runtime the user clicks the expander and the explorerbar "expands". The width when expanded keeps reverting back to 175, no matter how many times its changed. Frustrating...
The only way to programmatically change the width of the flyout is by handling NavigationPaneFlyoutDisplaying and setting the PreferredSize property of the event arguments. Presumably you are doing that somewhere without being aware of it.
[EDIT]I read this again, and it occurred to me you are probably talking about clicking the navigation pane expansion button...the same logic applies, but the event is NavigationPaneExpanding, and the event args property is PreferredWidth. That must be getting set somewhere.