Created attachment 17771 [details]
If you change device orientation on Android, all looks fine. If you do this on iOS, the status bar disappears and the navigation bar is smaller.
While looking at this, please also look at https://bugzilla.xamarin.com/show_bug.cgi?id=42651
Tested this on 18.104.22.168-pre2.
The behavior you're describing is the iOS default for iOS 8 or later in vertically compact environments:
This may be a good candidate for Platform Specifics; I'm marking it as an enhancement.
I'm presently in the midst of implementing this along with options for the update animation.
Should be fixed in 2.3.4-pre1. Please note that this is an opt-in behavior. Please see https://github.com/xamarin/Xamarin.Forms/pull/463 for more information. Thank you!