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Page navigation breaks if hardware back button pressed during page transition
Reproductible on Outlook sample by tapping Calendar in the Hub and quickly pressing hardware back button before Calendar appears.
Calendar will appear but next time back is tapped it will pop up the exit dialog instead of navigating back to the hub.
After peaking with the debugger, I guess the problem is prematurely setting currentNavigationKind to NavigationType.None in PageNavigator.CloseAndDisposePageNode() when the hub is hidden (we have HideThenShow transition mode in this case).
After that, the Screen invokes OnExitApplication() which will end up calling NavigateBack(). Which calls Hide() of Calendar, but the call will be silently ignored since it is not yet shown, but the navigator will discard the Calendar's page node.