Force iOS 13 to use Full Screen for Modal

- Once we are on xCode 11 and can account for swipe dismiss we can make this better

Author: PureWeen

Xamarin.Forms.Platform.iOS/Extensions/Extensions.cs

@@ -1,3 +1,4 @@
+using System;
 using UIKit;
 using Xamarin.Forms.Internals;
 
@@ -74,6 +75,19 @@ public static void ApplyKeyboard(this IUITextInputTraits textInput, Keyboard key
 			}
 		}
 
+		internal static UIModalPresentationStyle ToNativeModalPresentationStyle(this PlatformConfiguration.iOSSpecific.UIModalPresentationStyle style)
+		{
+			switch (style)
+			{
+				case PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FormSheet:
+					return UIModalPresentationStyle.FormSheet;
+				case PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FullScreen:
+					return UIModalPresentationStyle.FullScreen;
+				default:
+					throw new ArgumentOutOfRangeException(nameof(style));
+			}
+		}
+
 		internal static UIReturnKeyType ToUIReturnKeyType(this ReturnType returnType)
 		{
 			switch (returnType)

Xamarin.Forms.Platform.iOS/ModalWrapper.cs

@@ -14,8 +14,8 @@ internal ModalWrapper(IVisualElementRenderer modal)
 			_modal = modal;
 
 			var elementConfiguration = modal.Element as IElementConfiguration<Page>;
-			if (elementConfiguration?.On<PlatformConfiguration.iOS>().ModalPresentationStyle() == PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FormSheet)
-				ModalPresentationStyle = UIKit.UIModalPresentationStyle.FormSheet;
+			var modalPresentationStyle = elementConfiguration?.On<PlatformConfiguration.iOS>()?.ModalPresentationStyle() ?? PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FullScreen;
+			ModalPresentationStyle = modalPresentationStyle.ToNativeModalPresentationStyle();
 
 			View.BackgroundColor = UIColor.White;
 			View.AddSubview(modal.ViewController.View);