dotnet · PureWeen · Jul 30, 2026
diff --git a/...nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.Before.targets b/...nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.Before.targets
@@ -38,10 +38,9 @@
 
     BackendIdentity records the stable, MAUI-owned key for each built-in backend so
     the built-in platforms describe themselves through exactly the same registration
-    metadata that an external backend NuGet contributes. For recognized platform TFMs
-    activation still flows through TargetPlatformIdentifier(s); BackendIdentity here is
-    purely descriptive (built-ins do not declare an ActivationValue, so the neutral-TFM
-    activation branch never fires for them).
+    metadata that an external backend NuGet contributes. A registration can therefore
+    activate through TargetPlatformIdentifier(s) on a recognized platform TFM or through
+    MauiActiveBackend on a neutral TFM without repeating its backend identity.
   -->
   <ItemGroup>
     <MauiPlatformSpecificFolder Include="$(AndroidProjectFolder)" TargetPlatformIdentifier="android" BackendIdentity="android" />
@@ -59,10 +58,9 @@
       - ActivationValue is back-filled from BackendIdentity so a backend that
         only declares a stable identity key (BackendIdentity="gtk") can be
         activated for a neutral TFM via <MauiActiveBackend>gtk</MauiActiveBackend>
-        without having to repeat the value. Built-in platforms declare a
-        BackendIdentity but no ActivationValue is derived for them here because
-        they are TPI-recognized (see the guard below), so they never take the
-        neutral-activation branch.
+        without having to repeat the value. This also applies when the same
+        registration declares TargetPlatformIdentifier(s), giving it both recognized
+        TPI and neutral-TFM activation paths.
       - ActivationProperty defaults to the well-known MauiActiveBackend selector
         whenever an ActivationValue is present but no explicit property was named.
       - _MauiResolvedActivationValue captures the *current value* of the property
@@ -91,17 +89,10 @@
         <TargetPlatformIdentifiers>%(MauiPlatformSpecificFolder.TargetPlatformIdentifier)</TargetPlatformIdentifiers>
       </MauiPlatformSpecificFolder>
 
-      <!--
-        Back-fill ActivationValue from BackendIdentity ONLY for folders that are not
-        already TPI-recognized. Built-in platforms carry both a TargetPlatformIdentifier
-        and a BackendIdentity; deriving an ActivationValue for them would let a stray
-        MauiActiveBackend value pull an unrelated built-in folder into a neutral build.
-        Gating on empty TargetPlatformIdentifier(s) keeps the neutral-activation branch
-        exclusive to backends that actually rely on it.
-      -->
+      <!-- Preserve an explicitly authored ActivationValue; otherwise use the stable backend key. -->
       <MauiPlatformSpecificFolder
           Update="@(MauiPlatformSpecificFolder)"
-          Condition=" '%(MauiPlatformSpecificFolder.ActivationValue)' == '' and '%(MauiPlatformSpecificFolder.BackendIdentity)' != '' and '%(MauiPlatformSpecificFolder.TargetPlatformIdentifier)' == '' and '%(MauiPlatformSpecificFolder.TargetPlatformIdentifiers)' == '' ">
+          Condition=" '%(MauiPlatformSpecificFolder.ActivationValue)' == '' and '%(MauiPlatformSpecificFolder.BackendIdentity)' != '' ">
         <ActivationValue>%(MauiPlatformSpecificFolder.BackendIdentity)</ActivationValue>
       </MauiPlatformSpecificFolder>
 

diff --git a/....Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.targets b/....Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.targets
@@ -150,23 +150,32 @@
   <!-- Removals -->
     <ItemGroup>
       <!--
-        Remove every Compile item under $(PlatformsProjectFolder) that is not
-        part of the current build configuration AND has not been explicitly
+        Remove every Compile item under $(PlatformsProjectFolder) that is marked
+        as excluded from the current configuration AND has not been explicitly
         kept by _MauiCollectPlatformSpecificCompileItems above.
 
         The Condition references %(Compile.ExcludeFromCurrentConfiguration)
-        from inside an Include of a *different* item type — this is MSBuild
-        cross-item-type batching: the filesystem glob is evaluated once per
-        unique value of Compile.ExcludeFromCurrentConfiguration. The blanket
-        <Compile Update> initially marks every $(PlatformsProjectFolder)/**
-        file true, then active-platform updates flip matching items back to
-        false. Keep the Condition so only the true batch is removed; otherwise
-        active-platform files may be removed or inactive files may leak in.
+        from inside an Include of a *different* item type, preserving the
+        cross-item-type batching that isolates the true Compile metadata bucket.
+        Root the removal candidates in that batched @(Compile) list rather than
+        a fresh filesystem glob, which would also capture physical files from
+        false metadata buckets. The outside-platform helper then narrows the
+        candidates without mutating or reordering @(Compile).
       -->
       <_MauiPlatformCompileToRemove
           Condition=" '%(Compile.ExcludeFromCurrentConfiguration)' == 'true' "
-          Include="$(PlatformsProjectFolder)**/*$(DefaultLanguageSourceExtension)"
-          Exclude="@(_MauiPlatformSpecificCompileItems)" />
+          Include="@(Compile)" />
+      <_MauiCompileItemsOutsidePlatformFolder Include="@(_MauiPlatformCompileToRemove)" />
+      <_MauiCompileItemsOutsidePlatformFolder
+          Remove="$(PlatformsProjectFolder)**/*$(DefaultLanguageSourceExtension)" />
+      <_MauiPlatformCompileToRemove
+          Remove="@(_MauiCompileItemsOutsidePlatformFolder)"
+          MatchOnMetadata="FullPath"
+          MatchOnMetadataOptions="PathLike" />
+      <_MauiPlatformCompileToRemove
+          Remove="@(_MauiPlatformSpecificCompileItems)"
+          MatchOnMetadata="FullPath"
+          MatchOnMetadataOptions="PathLike" />
       <Compile Remove="@(_MauiPlatformCompileToRemove)" />
 
       <!-- Remove all Windows (WinUI) XAML Files from the Windows folder -->

diff --git a/src/Controls/tests/Xaml.UnitTests/MSBuild/MSBuildTests.cs b/src/Controls/tests/Xaml.UnitTests/MSBuild/MSBuildTests.cs
@@ -1410,6 +1410,112 @@ public static class After
 				StringComparison.OrdinalIgnoreCase);
 		}
 
+		// Regression for cross-item-type batching in _MauiRemovePlatformCompileItems.
+		// The true metadata batch must contain only the matching Compile identities;
+		// a fresh filesystem glob would also capture KeepMarker.cs from the false
+		// bucket and remove it because this test intentionally has no folder allow-list.
+		[Fact]
+		public void SingleProject_RemovePlatformCompileItems_RemovesOnlyCompileItemsMarkedTrue()
+		{
+			SetUp();
+			var project = NewElement("Project").WithAttribute("Sdk", "Microsoft.NET.Sdk");
+			var propertyGroup = NewElement("PropertyGroup");
+			propertyGroup.Add(NewElement("TargetFramework").WithValue(GetTfm()));
+			propertyGroup.Add(NewElement("SingleProject").WithValue("true"));
+			propertyGroup.Add(NewElement("EnableDefaultCompileItems").WithValue("false"));
+			project.Add(propertyGroup);
+			AddMauiReferences(project);
+			AddSingleProjectBeforeTargetsImport(project);
+
+			var compileItems = NewElement("ItemGroup");
+			compileItems.Add(NewElement("Compile").WithAttribute("Include", "Before.cs"));
+			compileItems.Add(NewElement("Compile").WithAttribute("Include", "Platforms\\Mixed\\KeepMarker.cs"));
+			compileItems.Add(NewElement("Compile").WithAttribute("Include", "Platforms\\Mixed\\RemoveMarker.cs"));
+			compileItems.Add(NewElement("Compile").WithAttribute("Include", "After.cs"));
+			project.Add(compileItems);
+
+			WriteFile("Before.cs", @"
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public static class Before
+{
+	public static string Value => ""Before"";
+}");
+
+			WriteFile("Platforms\\Mixed\\KeepMarker.cs", @"
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public static class KeepMarker
+{
+	public static string Value => ""Keep"";
+}");
+
+			WriteFile("Platforms\\Mixed\\RemoveMarker.cs", @"
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public static class RemoveMarker
+{
+	public static string Value => ""Remove"";
+}");
+
+			WriteFile("After.cs", @"
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public static class After
+{
+	public static string Value => ""After"";
+}");
+
+			AddSingleProjectTargetsImport(project);
+
+			// Apply the explicit metadata after the shipping targets import so the
+			// blanket Platforms/** update has already run.
+			var compileMetadata = NewElement("ItemGroup");
+			var keptCompile = NewElement("Compile").WithAttribute("Update", "Platforms\\Mixed\\KeepMarker.cs");
+			keptCompile.Add(NewElement("ExcludeFromCurrentConfiguration").WithValue("false"));
+			keptCompile.Add(NewElement("TestMetadata").WithValue("preserved"));
+			compileMetadata.Add(keptCompile);
+			var removedCompile = NewElement("Compile").WithAttribute("Update", "Platforms\\Mixed\\RemoveMarker.cs");
+			removedCompile.Add(NewElement("ExcludeFromCurrentConfiguration").WithValue("true"));
+			compileMetadata.Add(removedCompile);
+			project.Add(compileMetadata);
+
+			var dumpTarget = NewElement("Target")
+				.WithAttribute("Name", "_TestDumpCompileItemsAfterRemoval")
+				.WithAttribute("AfterTargets", "_MauiRemovePlatformCompileItems");
+			var dumpItems = NewElement("ItemGroup");
+			dumpItems.Add(NewElement("_TestKeptCompile")
+				.WithAttribute("Include", "@(Compile)")
+				.WithAttribute("Condition", " '%(Compile.Filename)' == 'KeepMarker' "));
+			dumpTarget.Add(dumpItems);
+			dumpTarget.Add(NewElement("Message")
+				.WithAttribute("Importance", "high")
+				.WithAttribute("Text", "COMPILE_ORDER: @(Compile->'%(Filename)', '|')"));
+			dumpTarget.Add(NewElement("Message")
+				.WithAttribute("Importance", "high")
+				.WithAttribute("Condition", " '%(Compile.Filename)' == 'KeepMarker' ")
+				.WithAttribute("Text", "KEEP_META: %(Compile.ExcludeFromCurrentConfiguration)|%(Compile.TestMetadata)"));
+			dumpTarget.Add(NewElement("Message")
+				.WithAttribute("Importance", "high")
+				.WithAttribute("Text", "KEEP_COUNT: @(_TestKeptCompile->Count())"));
+			project.Add(dumpTarget);
+
+			var projectFile = IOPath.Combine(tempDirectory, "test.csproj");
+			project.Save(projectFile);
+
+			var log = Build(projectFile);
+
+			var testDll = IOPath.Combine(intermediateDirectory, "test.dll");
+			AssertExists(testDll, nonEmpty: true);
+			AssertTypeExists(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.Before");
+			AssertTypeExists(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.KeepMarker");
+			AssertTypeDoesNotExist(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.RemoveMarker");
+			AssertTypeExists(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.After");
+			Assert.Contains("COMPILE_ORDER: Before|KeepMarker|After", log, StringComparison.OrdinalIgnoreCase);
+			Assert.Contains("KEEP_META: false|preserved", log, StringComparison.OrdinalIgnoreCase);
+			Assert.Contains("KEEP_COUNT: 1", log, StringComparison.OrdinalIgnoreCase);
+		}
+
 		// Backward compatibility: a folder that declares only the legacy singular
 		// TargetPlatformIdentifier metadata must continue to match exactly that TPI.
 		[Theory]
@@ -1465,6 +1571,73 @@ public static class LegacyIosMarker
 				AssertTypeDoesNotExist(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.LegacyIosMarker");
 		}
 
+		// One backend registration may serve both a recognized TPI and a neutral TFM.
+		// BackendIdentity defaults the neutral ActivationValue even when TPI metadata
+		// is present, while the neutral selector remains ignored on another recognized TPI.
+		[Theory]
+		[InlineData("macos", "", true)]
+		[InlineData("", "macos", true)]
+		[InlineData("android", "macos", false)]
+		[InlineData("android", "", false)]
+		[InlineData("", "gtk", false)]
+		public void SingleProject_BackendRegistrationSupportsRecognizedAndNeutralActivation(
+			string targetPlatformIdentifier,
+			string activeBackend,
+			bool shouldIncludeMacOsFile)
+		{
+			SetUp();
+			var project = NewElement("Project").WithAttribute("Sdk", "Microsoft.NET.Sdk");
+			var propertyGroup = NewElement("PropertyGroup");
+			propertyGroup.Add(NewElement("TargetFramework").WithValue(GetTfm()));
+			propertyGroup.Add(NewElement("SingleProject").WithValue("true"));
+			project.Add(propertyGroup);
+			AddMauiReferences(project);
+			AddSingleProjectBeforeTargetsImport(project);
+
+			var customMappings = NewElement("ItemGroup");
+			customMappings.Add(NewElement("MauiPlatformSpecificFolder")
+				.WithAttribute("Include", "Platforms\\MacOS\\")
+				.WithAttribute("TargetPlatformIdentifiers", "macos")
+				.WithAttribute("BackendIdentity", "macos"));
+			project.Add(customMappings);
+
+			WriteFile("Entry.cs", @"
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public static class Entry
+{
+	public static string Value => ""ok"";
+}");
+
+			WriteFile("Platforms\\MacOS\\MacOsMarker.cs", @"
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public static class MacOsMarker
+{
+	public static string Value => ""MacOS"";
+}");
+
+			AddSingleProjectTargetsImport(project);
+
+			var projectFile = IOPath.Combine(tempDirectory, "test.csproj");
+			project.Save(projectFile);
+
+			var args = "";
+			if (!string.IsNullOrEmpty(targetPlatformIdentifier))
+				args += $"-p:_SingleProjectTestTargetPlatformIdentifier={targetPlatformIdentifier}";
+			if (!string.IsNullOrEmpty(activeBackend))
+				args += $" -p:MauiActiveBackend={activeBackend}";
+			Build(projectFile, additionalArgs: args);
+
+			var testDll = IOPath.Combine(intermediateDirectory, "test.dll");
+			AssertExists(testDll, nonEmpty: true);
+
+			if (shouldIncludeMacOsFile)
+				AssertTypeExists(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.MacOsMarker");
+			else
+				AssertTypeDoesNotExist(testDll, "Microsoft.Maui.Controls.Xaml.UnitTests.MacOsMarker");
+		}
+
 		// Neutral-TFM activation (the GTK scenario from #35021/#36650). On a plain
 		// net11.0 inner build no TargetPlatformIdentifier is recognized; a backend
 		// declares a stable BackendIdentity and is activated via the well-known
@@ -1659,6 +1832,7 @@ public void SingleProject_CustomActivationPropertyAndValueActivateBackend(string
 			var customMappings = NewElement("ItemGroup");
 			customMappings.Add(NewElement("MauiPlatformSpecificFolder")
 				.WithAttribute("Include", "Platforms\\Foo\\")
+				.WithAttribute("TargetPlatformIdentifiers", "foo")
 				.WithAttribute("BackendIdentity", "foo")
 				.WithAttribute("ActivationProperty", "MyBackendSwitch")
 				.WithAttribute("ActivationValue", "on"));