[UWP] Prevent Layouts with no background color from being input transparent (#4508)

* [UWP] Prevent Layouts with no background color from being input transparent; fixes #4360;

* Add comments

* Fix build for UITests projects

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue4360.xaml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Xamarin.Forms.Controls.Issues.Issue4360">
+    <ContentPage.Content>
+       <StackLayout HorizontalOptions="FillAndExpand" HeightRequest="50">
+            <StackLayout.GestureRecognizers>
+                <TapGestureRecognizer Tapped="TapGestureRecognizer_OnTapped"/>
+            </StackLayout.GestureRecognizers>
+            
+            <Label x:Name="Label" BackgroundColor="Red" VerticalOptions="FillAndExpand" VerticalTextAlignment="Center" 
+					Text="Tap next to this label (not on it); If the text changes to 'Success', the test has passed." 
+					HorizontalOptions="Center" />
+        </StackLayout>
+    </ContentPage.Content>
+</ContentPage>

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue4360.xaml.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+using Xamarin.Forms;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using Xamarin.Forms.Xaml;
+
+namespace Xamarin.Forms.Controls.Issues
+{
+#if APP
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 4360, "UWP: TapGestureRecognizer works on Layout only if BackgroundColor is set", 
+		PlatformAffected.UWP)]
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	public partial class Issue4360 : ContentPage
+	{
+		const string Success = "Success";
+
+		public Issue4360()
+		{
+
+			InitializeComponent();
+		}
+
+		void TapGestureRecognizer_OnTapped(object sender, EventArgs e)
+		{
+			Label.Text = Success;
+		}
+
+	}
+#endif
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -412,6 +412,10 @@
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue4136.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue4262.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue4360.xaml.cs">
+      <DependentUpon>Issue4360.xaml</DependentUpon>
+      <SubType>Code</SubType>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)LegacyComponents\NonAppCompatSwitch.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MapsModalCrash.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ModalActivityIndicatorTest.cs" />
@@ -1058,4 +1062,10 @@
       <Generator>MSBuild:Compile</Generator>
     </EmbeddedResource>
   </ItemGroup>
-</Project>
+  <ItemGroup>
+    <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue4360.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </EmbeddedResource>
+  </ItemGroup>
+</Project>

Xamarin.Forms.Platform.UAP/VisualElementRenderer.cs

@@ -617,6 +617,19 @@ namespace Xamarin.Forms.Platform.UWP
 			{
 				RemoveBackgroundLayer();
 				IsHitTestVisible = Element.IsEnabled && !Element.InputTransparent;
+
+				if (!IsHitTestVisible)
+				{
+					return;
+				}
+
+				// If this Panel's background brush is null, the UWP considers it transparent to hit testing (even 
+				// when IsHitTestVisible is true). So we have to explicitly set a background brush to make it show up
+				// in hit testing. 
+				if (Element is Layout && Background == null)
+				{
+					Background = new SolidColorBrush(Windows.UI.Colors.Transparent);
+				}
 			}
 		}