A Double value that specifies the window width, in pixels, at which the menu is automatically displayed in the Flyout state. The default is 720.
The HamburgerMenu supports the adaptive layout, thus it is displayed in the view state that corresponds to the window size. Using the HamburgerMenu.AvailableViewStates property, you can specify the view states in which the menu can be displayed. The MinimizedFlyoutStateFrameThreshold property specifies the window width, in pixels, at which the menu automatically turns into the Flyout state.