I am unable to set this property to true. According to the docs, true is the default value. If I just create instance of NavigationBar in a new user control, and look at CanExpandMultiple in the VS properties window, I can see that it is unchecked. If I check it, then this gets added to the markup:
But -- the checkbox in the VS properties window is immediately unchecked! I know that sounds weird, but it's happening.
At runtime I am unable to expand more than one pane, so the added markup is having no effect.