fix: Only check Background on specific elements when hit-testing

Align with WinUI, where the presence of a non-null Background only causes a hit-test to pass when it's set on Border, Panel, or ContentPresenter.

In particular this means that Controls with Background set will not themselves intercept pointer events; only elements within their templates will intercept pointers.

src/Uno.UI/UI/Xaml/Controls/Border/Border.cs

@@ -30,6 +30,7 @@
 using Color = System.Drawing.Color;
 using View = Windows.UI.Xaml.UIElement;
 #endif
+using _Debug = System.Diagnostics.Debug;
 
 namespace Windows.UI.Xaml.Controls
 {
@@ -296,5 +297,17 @@ protected virtual void OnBorderBrushChanged(Brush oldValue, Brush newValue)
 		#endregion
 
 		internal override bool CanHaveChildren() => true;
+
+		internal override bool IsViewHit() => IsViewHitImpl(this);
+
+		internal static bool IsViewHitImpl(FrameworkElement element)
+		{
+			_Debug.Assert(element is Panel
+				|| element is Border
+				|| element is ContentPresenter
+			);
+
+			return element.Background != null;
+		}
 	}
 }

src/Uno.UI/UI/Xaml/Controls/Border/Border.skia.cs

@@ -95,8 +95,6 @@ protected override void OnBackgroundChanged(DependencyPropertyChangedEventArgs a
 			UpdateBorder();
 		}
 
-		internal override bool IsViewHit()
-			=> Background != null || base.IsViewHit();
 		bool ICustomClippingElement.AllowClippingToLayoutSlot => !(Child is UIElement ue) || ue.RenderTransform == null;
 		bool ICustomClippingElement.ForceClippingToLayoutSlot => CornerRadius != CornerRadius.None;
 	}

src/Uno.UI/UI/Xaml/Controls/ContentPresenter/ContentPresenter.cs

@@ -1102,5 +1102,7 @@ protected override Size MeasureOverride(Size size)
 		private protected override Thickness GetBorderThickness() => BorderThickness;
 
 		internal override bool CanHaveChildren() => true;
+
+		internal override bool IsViewHit() => Border.IsViewHitImpl(this);
 	}
 }

src/Uno.UI/UI/Xaml/Controls/Panel/Panel.cs

@@ -184,5 +184,6 @@ private protected void OnBackgroundSizingChangedInnerPanel(DependencyPropertyCha
 
 		partial void UpdateBorder();
 
+		internal override bool IsViewHit() => Border.IsViewHitImpl(this);
 	}
 }

src/Uno.UI/UI/Xaml/FrameworkElement.cs

@@ -794,8 +794,7 @@ private bool IsHeightConstrained(View requester)
 			return this.IsHeightConstrainedSimple();
 		}
 
-		internal override bool IsViewHit()
-			=> Background != null;
+		internal override bool IsViewHit() => false;
 
 		/// <summary>
 		/// The list of available children render phases, if this