Address review: reset container on disconnect, make validation hook protected

Two defects found in review of the SetContainerView extensibility point.

1. Disconnecting a handler that still had an active container left both
   ContainerView and HasContainer set. ElementHandler.DisconnectHandler()
   nulls PlatformView and never touched container state, so a later
   reconnect hit the HasContainer equality short-circuit in the setter,
   skipped SetupContainer(), and kept serving the stale wrapper that
   still held the previous platform view.

   The teardown has to run while PlatformView is still reachable: the
   generic PlatformView getter throws when null, so calling
   RemoveContainer() after the null would throw on Android and Tizen.
   Add a private protected virtual ElementHandler.OnDisconnecting() hook
   that runs before PlatformView is cleared, and override it in
   ViewHandler to flip HasContainer off (which unwinds the container
   through the platform's own RemoveContainer()) and clear ContainerView.
   RemoveContainer() is therefore invoked under exactly the precondition
   it already runs under during normal operation, so every existing
   platform override stays on its supported path.

2. ValidateContainerView was private protected, so an external subclass
   that shadows ContainerView with its own wrapper type could not enforce
   that type and the broad setter could corrupt its strongly typed
   getter. Promote it to protected virtual with XML docs, and update the
   iOS/MacCatalyst and Tizen overrides accordingly.

The external backend test handler now overrides ValidateContainerView to
require its own wrapper type. Note the override is declared with `object`
because on the neutral TFM the PlatformView alias is System.Object.

New regression tests (5), all verified to fail without the fixes:
DisconnectingWithAnActiveContainerTearsTheContainerDown,
ReconnectingAfterDisconnectWithAnActiveContainerRebuildsTheContainer,
SetContainerViewRejectsAContainerOfTheWrongType,
SetContainerViewRejectionLeavesAnExistingContainerIntact,
SetContainerViewStillAcceptsNullAfterValidationIsOverridden.

Also applies two reviewer nits: scope the SetupContainer/RemoveContainer
remarks to overrides outside this assembly (in-box handlers still use the
private protected setter), and restore `using System;` first in
ViewHandlerOfT.iOS.cs to match the other platform partials.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: Redth

src/Core/src/Handlers/Element/ElementHandler.cs

@@ -141,6 +141,10 @@ void IElementHandler.DisconnectHandler()
 		{
 			if (PlatformView != null && VirtualView != null)
 			{
+				// Give derived handlers a chance to tear down state that needs the platform view
+				// while it is still reachable through the PlatformView property.
+				OnDisconnecting();
+
 				// We set the PlatformView to null so no one outside of this handler tries to access
 				// PlatformView. PlatformView access should be isolated to the instance passed into
 				// DisconnectHandler
@@ -151,5 +155,10 @@ void IElementHandler.DisconnectHandler()
 
 			_handlerState = ElementHandlerState.Disconnected;
 		}
+
+		// Runs while PlatformView is still set, so overrides can use it to unwind platform state.
+		private protected virtual void OnDisconnecting()
+		{
+		}
 	}
 }

src/Core/src/Handlers/View/ViewHandler.cs

@@ -145,14 +145,17 @@ public virtual bool NeedsContainer
 		/// Constructs the <see cref="ContainerView"/> and adds <see cref="PlatformView"/> to a container.
 		/// </summary>
 		/// <remarks>This method is called when <see cref="HasContainer"/> is set to <see langword="true"/>.
-		/// Overrides should call <see cref="SetContainerView(PlatformView?)"/> to publish the container they created.</remarks>
+		/// An override that cannot reach the <see cref="ContainerView"/> setter - that is, one declared outside of this
+		/// assembly - should publish the container it created by calling <see cref="SetContainerView(PlatformView?)"/>.</remarks>
 		protected abstract void SetupContainer();
 
 		/// <summary>
 		/// Deconstructs the <see cref="ContainerView"/> and removes <see cref="PlatformView"/> from its container. 
 		/// </summary>
 		/// <remarks>This method is called when <see cref="HasContainer"/> is set to <see langword="false"/>.
-		/// Overrides should call <see cref="SetContainerView(PlatformView?)"/> with <see langword="null"/> to clear the container they removed.</remarks>
+		/// An override that cannot reach the <see cref="ContainerView"/> setter - that is, one declared outside of this
+		/// assembly - should clear the container it removed by calling <see cref="SetContainerView(PlatformView?)"/> with
+		/// <see langword="null"/>.</remarks>
 		protected abstract void RemoveContainer();
 
 		/// <summary>
@@ -232,10 +235,55 @@ protected void SetContainerView(PlatformView? containerView)
 			ContainerView = containerView;
 		}
 
-		// Lets a platform-specific handler reject container views that would break the strongly typed
-		// ContainerView property it shadows (for example the WrapperView-typed property on iOS and Tizen).
-		private protected virtual void ValidateContainerView(PlatformView containerView)
+		/// <summary>
+		/// Validates that <paramref name="containerView"/> is a container type this handler supports, throwing if it is not.
+		/// </summary>
+		/// <param name="containerView">The candidate container view. This is never <see langword="null"/>.</param>
+		/// <remarks>
+		/// <para>Called by <see cref="SetContainerView(PlatformView?)"/> before the value is stored. Override this when a derived
+		/// handler shadows <see cref="ContainerView"/> with a more specific type, so that an unsupported container is rejected
+		/// where the mistake is made instead of surfacing later as an <see cref="System.InvalidCastException"/> from the
+		/// shadowing getter.</para>
+		/// <para>The .NET MAUI iOS, Mac Catalyst and Tizen handlers use this to require a
+		/// <see cref="Platform.WrapperView"/>. An external backend that shadows <see cref="ContainerView"/> with its own
+		/// wrapper type should do the same.</para>
+		/// <example>
+		/// <code language="csharp">
+		/// public new MyWrapperView? ContainerView => (MyWrapperView?)base.ContainerView;
+		///
+		/// protected override void ValidateContainerView(MyPlatformView containerView)
+		/// {
+		///     if (containerView is not MyWrapperView)
+		///     {
+		///         throw new ArgumentException(
+		///             $"The container view must be a {nameof(MyWrapperView)}.",
+		///             nameof(containerView));
+		///     }
+		///
+		///     base.ValidateContainerView(containerView);
+		/// }
+		/// </code>
+		/// </example>
+		/// </remarks>
+		/// <exception cref="System.ArgumentException">Thrown by overrides when <paramref name="containerView"/> is not supported.</exception>
+		protected virtual void ValidateContainerView(PlatformView containerView)
+		{
+		}
+
+		private protected override void OnDisconnecting()
 		{
+			// The platform view is still reachable here, which is the same precondition RemoveContainer()
+			// runs under during normal operation, so existing overrides stay on their supported path.
+			// Without this, ContainerView and HasContainer would survive the disconnect and a later
+			// reconnect would skip SetupContainer() and keep pointing at the stale container.
+			if (HasContainer)
+			{
+				HasContainer = false;
+			}
+
+			ContainerView = null;
+
+			base.OnDisconnecting();
 		}
 
 		object? IViewHandler.ContainerView => ContainerView;

src/Core/src/Handlers/View/ViewHandlerOfT.Tizen.cs

@@ -21,14 +21,17 @@ public abstract partial class ViewHandler<TVirtualView, TPlatformView> : IPlatfo
 			protected set => base.ContainerView = value;
 		}
 
-		private protected override void ValidateContainerView(NView containerView)
+		/// <inheritdoc/>
+		protected override void ValidateContainerView(NView containerView)
 		{
 			if (containerView is not WrapperView)
 			{
 				throw new ArgumentException(
 					$"The container view must be a {nameof(WrapperView)} because {GetType().Name} exposes {nameof(ContainerView)} as a {nameof(WrapperView)}.",
 					nameof(containerView));
 			}
+
+			base.ValidateContainerView(containerView);
 		}
 
 		~ViewHandler()

src/Core/src/Handlers/View/ViewHandlerOfT.iOS.cs

@@ -1,5 +1,5 @@
-using Microsoft.Maui.Graphics;
 using System;
+using Microsoft.Maui.Graphics;
 using UIKit;
 
 namespace Microsoft.Maui.Handlers
@@ -12,14 +12,17 @@ public partial class ViewHandler<TVirtualView, TPlatformView> : IPlatformViewHan
 			protected set => base.ContainerView = value;
 		}
 
-		private protected override void ValidateContainerView(UIView containerView)
+		/// <inheritdoc/>
+		protected override void ValidateContainerView(UIView containerView)
 		{
 			if (containerView is not WrapperView)
 			{
 				throw new ArgumentException(
 					$"The container view must be a {nameof(WrapperView)} because {GetType().Name} exposes {nameof(ContainerView)} as a {nameof(WrapperView)}.",
 					nameof(containerView));
 			}
+
+			base.ValidateContainerView(containerView);
 		}
 
 		public UIViewController? ViewController { get; set; }

src/Core/src/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -364,3 +364,4 @@ Microsoft.Maui.ISwipeItemMenuItemIconColor
 Microsoft.Maui.ISwipeItemMenuItemIconColor.IconColor.get -> Microsoft.Maui.Graphics.Color?
 static Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.MapIconColor(Microsoft.Maui.Handlers.ISwipeItemMenuItemHandler! handler, Microsoft.Maui.ISwipeItemMenuItem! view) -> void
 Microsoft.Maui.Handlers.ViewHandler.SetContainerView(Android.Views.View? containerView) -> void
+virtual Microsoft.Maui.Handlers.ViewHandler.ValidateContainerView(Android.Views.View! containerView) -> void

src/Core/src/PublicAPI/net-ios/PublicAPI.Unshipped.txt

@@ -64,3 +64,5 @@ Microsoft.Maui.ISwipeItemMenuItemIconColor
 Microsoft.Maui.ISwipeItemMenuItemIconColor.IconColor.get -> Microsoft.Maui.Graphics.Color?
 static Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.MapIconColor(Microsoft.Maui.Handlers.ISwipeItemMenuItemHandler! handler, Microsoft.Maui.ISwipeItemMenuItem! view) -> void
 Microsoft.Maui.Handlers.ViewHandler.SetContainerView(UIKit.UIView? containerView) -> void
+virtual Microsoft.Maui.Handlers.ViewHandler.ValidateContainerView(UIKit.UIView! containerView) -> void
+override Microsoft.Maui.Handlers.ViewHandler<TVirtualView, TPlatformView>.ValidateContainerView(UIKit.UIView! containerView) -> void

src/Core/src/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt

@@ -61,3 +61,5 @@ Microsoft.Maui.ISwipeItemMenuItemIconColor
 Microsoft.Maui.ISwipeItemMenuItemIconColor.IconColor.get -> Microsoft.Maui.Graphics.Color?
 static Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.MapIconColor(Microsoft.Maui.Handlers.ISwipeItemMenuItemHandler! handler, Microsoft.Maui.ISwipeItemMenuItem! view) -> void
 Microsoft.Maui.Handlers.ViewHandler.SetContainerView(UIKit.UIView? containerView) -> void
+virtual Microsoft.Maui.Handlers.ViewHandler.ValidateContainerView(UIKit.UIView! containerView) -> void
+override Microsoft.Maui.Handlers.ViewHandler<TVirtualView, TPlatformView>.ValidateContainerView(UIKit.UIView! containerView) -> void

src/Core/src/PublicAPI/net-tizen/PublicAPI.Unshipped.txt

@@ -22,3 +22,5 @@ Microsoft.Maui.ISwipeItemMenuItemIconColor
 Microsoft.Maui.ISwipeItemMenuItemIconColor.IconColor.get -> Microsoft.Maui.Graphics.Color?
 static Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.MapIconColor(Microsoft.Maui.Handlers.ISwipeItemMenuItemHandler! handler, Microsoft.Maui.ISwipeItemMenuItem! view) -> void
 Microsoft.Maui.Handlers.ViewHandler.SetContainerView(Tizen.NUI.BaseComponents.View? containerView) -> void
+virtual Microsoft.Maui.Handlers.ViewHandler.ValidateContainerView(Tizen.NUI.BaseComponents.View! containerView) -> void
+override Microsoft.Maui.Handlers.ViewHandler<TVirtualView, TPlatformView>.ValidateContainerView(Tizen.NUI.BaseComponents.View! containerView) -> void

src/Core/src/PublicAPI/net-windows/PublicAPI.Unshipped.txt

@@ -49,3 +49,4 @@ Microsoft.Maui.ISwipeItemMenuItemIconColor
 Microsoft.Maui.ISwipeItemMenuItemIconColor.IconColor.get -> Microsoft.Maui.Graphics.Color?
 static Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.MapIconColor(Microsoft.Maui.Handlers.ISwipeItemMenuItemHandler! handler, Microsoft.Maui.ISwipeItemMenuItem! view) -> void
 Microsoft.Maui.Handlers.ViewHandler.SetContainerView(Microsoft.UI.Xaml.FrameworkElement? containerView) -> void
+virtual Microsoft.Maui.Handlers.ViewHandler.ValidateContainerView(Microsoft.UI.Xaml.FrameworkElement! containerView) -> void

src/Core/src/PublicAPI/net/PublicAPI.Unshipped.txt

@@ -32,3 +32,4 @@ static Microsoft.Maui.Handlers.ShapeViewHandler.MapFlowDirection(Microsoft.Maui.
 static Microsoft.Maui.Handlers.SearchBarHandler.MapCursorPosition(Microsoft.Maui.Handlers.ISearchBarHandler! handler, Microsoft.Maui.ISearchBar! searchBar) -> void
 static Microsoft.Maui.Handlers.SearchBarHandler.MapSelectionLength(Microsoft.Maui.Handlers.ISearchBarHandler! handler, Microsoft.Maui.ISearchBar! searchBar) -> void
 Microsoft.Maui.Handlers.ViewHandler.SetContainerView(object? containerView) -> void
+virtual Microsoft.Maui.Handlers.ViewHandler.ValidateContainerView(object! containerView) -> void