[Extensibility] SingleProject: Register neutral-TFM backends

src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.Before.targets

@@ -28,14 +28,124 @@
     <TizenProjectFolder>$([MSBuild]::EnsureTrailingSlash('$(TizenProjectFolder)'))</TizenProjectFolder>
   </PropertyGroup>
 
+  <!--
+    Built-in folder mappings declare only the legacy singular TargetPlatformIdentifier
+    metadata so that _MauiNormalizePlatformSpecificFolders is the single source of
+    truth for back-filling the plural TargetPlatformIdentifiers contract. Keeping
+    one source avoids drift between the two metadata fields and ensures the
+    normalization code path is exercised on every SingleProject build (so a
+    regression in the back-fill is caught immediately).
+
+    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).
+  -->
   <ItemGroup>
-    <MauiPlatformSpecificFolder Include="$(AndroidProjectFolder)" TargetPlatformIdentifier="android" />
-    <MauiPlatformSpecificFolder Include="$(iOSProjectFolder)" TargetPlatformIdentifier="ios" />
-    <MauiPlatformSpecificFolder Include="$(MacCatalystProjectFolder)" TargetPlatformIdentifier="maccatalyst" />
-    <MauiPlatformSpecificFolder Include="$(WindowsProjectFolder)" TargetPlatformIdentifier="windows" />
-    <MauiPlatformSpecificFolder Include="$(TizenProjectFolder)" TargetPlatformIdentifier="tizen" />
+    <MauiPlatformSpecificFolder Include="$(AndroidProjectFolder)" TargetPlatformIdentifier="android" BackendIdentity="android" />
+    <MauiPlatformSpecificFolder Include="$(iOSProjectFolder)" TargetPlatformIdentifier="ios" BackendIdentity="ios" />
+    <MauiPlatformSpecificFolder Include="$(MacCatalystProjectFolder)" TargetPlatformIdentifier="maccatalyst" BackendIdentity="maccatalyst" />
+    <MauiPlatformSpecificFolder Include="$(WindowsProjectFolder)" TargetPlatformIdentifier="windows" BackendIdentity="windows" />
+    <MauiPlatformSpecificFolder Include="$(TizenProjectFolder)" TargetPlatformIdentifier="tizen" BackendIdentity="tizen" />
   </ItemGroup>
 
+  <!--
+    Normalize MauiPlatformSpecificFolder items so downstream targets and any
+    third-party extension can rely on a uniform shape:
+      - TargetPlatformIdentifiers is back-filled from the legacy singular
+        TargetPlatformIdentifier when only the singular is set.
+      - 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.
+      - 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
+        named by ActivationProperty. This is the neutral-TFM (non-TPI) activation
+        signal consumed by _MauiCollectPlatformSpecificCompileItems.
+    Identity (folder path) is intentionally not rewritten here because MSBuild
+    item-Identity rewriting requires a remove/re-add dance that loses caller
+    metadata. Instead, downstream consumers apply EnsureTrailingSlash() at the
+    use site (see _MauiCollectPlatformSpecificCompileItems).
+  -->
+  <!--
+    BeforeTargets is intentionally limited to _MauiRemovePlatformCompileItems
+    because _MauiCollectPlatformSpecificCompileItems already declares
+    DependsOnTargets="_MauiNormalizePlatformSpecificFolders" (single source of
+    truth for that ordering). The Remove target does not depend on Collect (so
+    Normalize-Before-Remove is the only way to guarantee Remove sees the
+    back-filled TargetPlatformIdentifiers metadata when Collect is short-circuited).
+  -->
+  <Target Name="_MauiNormalizePlatformSpecificFolders"
+          BeforeTargets="_MauiRemovePlatformCompileItems"
+          Condition=" '$(SingleProject)' == 'true' ">
+    <ItemGroup>
+      <MauiPlatformSpecificFolder
+          Update="@(MauiPlatformSpecificFolder)"
+          Condition=" '%(MauiPlatformSpecificFolder.TargetPlatformIdentifiers)' == '' and '%(MauiPlatformSpecificFolder.TargetPlatformIdentifier)' != '' ">
+        <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.
+      -->
+      <MauiPlatformSpecificFolder
+          Update="@(MauiPlatformSpecificFolder)"
+          Condition=" '%(MauiPlatformSpecificFolder.ActivationValue)' == '' and '%(MauiPlatformSpecificFolder.BackendIdentity)' != '' and '%(MauiPlatformSpecificFolder.TargetPlatformIdentifier)' == '' and '%(MauiPlatformSpecificFolder.TargetPlatformIdentifiers)' == '' ">
+        <ActivationValue>%(MauiPlatformSpecificFolder.BackendIdentity)</ActivationValue>
+      </MauiPlatformSpecificFolder>
+
+      <!-- MauiActiveBackend is the well-known neutral-TFM selector; default to it when unset. -->
+      <MauiPlatformSpecificFolder
+          Update="@(MauiPlatformSpecificFolder)"
+          Condition=" '%(MauiPlatformSpecificFolder.ActivationProperty)' == '' and '%(MauiPlatformSpecificFolder.ActivationValue)' != '' ">
+        <ActivationProperty>MauiActiveBackend</ActivationProperty>
+      </MauiPlatformSpecificFolder>
+
+      <!--
+        Resolve the *value* of the property named by ActivationProperty into a
+        dedicated metadata field. The '$(%(...))' form is MSBuild dynamic property
+        dereference: '%(...ActivationProperty)' expands to the property name first
+        (e.g. MauiActiveBackend), then '$(...)' expands that property's value. This
+        supports both the default MauiActiveBackend selector and a custom
+        ActivationProperty a backend NuGet may prefer. An unset property resolves
+        to empty and simply fails to activate, so a non-selected backend is excluded.
+      -->
+      <MauiPlatformSpecificFolder
+          Update="@(MauiPlatformSpecificFolder)"
+          Condition=" '%(MauiPlatformSpecificFolder.ActivationProperty)' != '' ">
+        <_MauiResolvedActivationValue>$(%(MauiPlatformSpecificFolder.ActivationProperty))</_MauiResolvedActivationValue>
+      </MauiPlatformSpecificFolder>
+
+      <!--
+        Pre-compute whitespace-stripped, lower-cased forms of the wanted
+        ActivationValue and the resolved property value so the activation branch of
+        _MauiCollectPlatformSpecificCompileItems is a plain metadata equality check.
+        Doing the Regex.Replace/ToLowerInvariant normalization here (as metadata
+        values) rather than inline in the collect Condition keeps that Condition
+        parseable — nesting property functions that reference metadata on both sides
+        of an '==' inside a single item Condition trips MSBuild's condition parser
+        (MSB4092). Whitespace tolerance and case-insensitivity mirror the TPI match.
+      -->
+      <MauiPlatformSpecificFolder
+          Update="@(MauiPlatformSpecificFolder)"
+          Condition=" '%(MauiPlatformSpecificFolder.ActivationValue)' != '' ">
+        <_MauiActivationValueNormalized>$([System.Text.RegularExpressions.Regex]::Replace('%(MauiPlatformSpecificFolder.ActivationValue)', '\s+', '').ToLowerInvariant())</_MauiActivationValueNormalized>
+        <_MauiResolvedActivationValueNormalized>$([System.Text.RegularExpressions.Regex]::Replace('%(MauiPlatformSpecificFolder._MauiResolvedActivationValue)', '\s+', '').ToLowerInvariant())</_MauiResolvedActivationValueNormalized>
+      </MauiPlatformSpecificFolder>
+    </ItemGroup>
+  </Target>
+
   <PropertyGroup Condition=" '$(SingleProject)' == 'true' and '$([MSBuild]::GetTargetPlatformIdentifier($(TargetFramework)))' == 'android' ">
     <AndroidManifest Condition=" Exists('$(AndroidProjectFolder)AndroidManifest.xml') ">$(AndroidProjectFolder)AndroidManifest.xml</AndroidManifest>
     <MonoAndroidResourcePrefix>$(AndroidProjectFolder)Resources</MonoAndroidResourcePrefix>

src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.targets

@@ -35,6 +35,109 @@
     </Compile>
   </ItemGroup>
 
+  <!--
+    Build the per-platform allow-list of Compile items that should survive the
+    blanket Platforms/** removal performed by _MauiRemovePlatformCompileItems.
+
+    Identity normalization: EnsureTrailingSlash() guards against the
+    foot-gun where a user-supplied item like
+        <MauiPlatformSpecificFolder Include="Platforms\Apple" .../>
+    (no trailing separator) would expand to the glob "Platforms\Apple**/*.cs"
+    and silently match sibling folders such as "Platforms\AppleX\" or
+    "Platforms\AppleLegacy\".
+
+    Inclusion contract — a folder is kept when ANY of:
+      (A) It declares neither a TargetPlatformIdentifiers list nor any backend
+          identity/activation metadata. This is the legacy "always include" /
+          condition-gated folder (its MauiPlatformSpecificFolder item Condition
+          already decided participation at evaluation time). Malformed backend
+          metadata that still carries ActivationProperty or BackendIdentity fails
+          closed instead of leaking backend files into every target framework.
+          A bare ActivationValue="$(UnsetProperty)" expands to empty during project
+          evaluation and is therefore indistinguishable from a legacy shared folder;
+          conditional backends must also declare ActivationProperty or BackendIdentity.
+      (B) TargetPlatformIdentifiers is a non-empty ';'-delimited list that
+          contains the current recognized $(TargetPlatformIdentifier). Comparison
+          is case-insensitive and tolerates any whitespace (ASCII space, tab, CR,
+          LF) around each entry; the Regex.Replace(\s+, '') strip is deliberate,
+          not Replace(' ', ''), so values formatted across multiple lines or
+          copy-pasted with tabs continue to match. The '$(TargetPlatformIdentifier)'
+          != '' guard prevents non-empty lists from accidentally matching during
+          non-platform (e.g. netstandard / design-time) builds where
+          $(TargetPlatformIdentifier) is empty.
+      (C) Neutral-TFM activation: the folder declares an ActivationValue and the
+          value of its ActivationProperty (resolved+normalized during normalization
+          into _MauiResolvedActivationValueNormalized, default property
+          MauiActiveBackend) matches the normalized ActivationValue. This is the path
+          a backend like GTK takes on a plain net11.0 inner build where no
+          TargetPlatformIdentifier is recognized. The empty TargetPlatformIdentifier
+          guard prevents a neutral backend selector from adding its files to a
+          recognized Android/iOS/etc. inner build. The normalization target already
+          stripped whitespace and lower-cased both sides, so this is a plain equality.
+  -->
+  <Target Name="_MauiCollectPlatformSpecificCompileItems"
+          DependsOnTargets="_MauiNormalizePlatformSpecificFolders"
+          BeforeTargets="_MauiRemovePlatformCompileItems"
+          Condition=" '$(SingleProject)' == 'true' and '@(MauiPlatformSpecificFolder)' != '' ">
+    <ItemGroup>
+      <_MauiPlatformSpecificCompileItems
+          Include="$([MSBuild]::EnsureTrailingSlash('%(MauiPlatformSpecificFolder.Identity)'))**/*$(DefaultLanguageSourceExtension)"
+          Condition=" ('%(MauiPlatformSpecificFolder.TargetPlatformIdentifiers)' == '' and '%(MauiPlatformSpecificFolder.ActivationValue)' == '' and '%(MauiPlatformSpecificFolder.BackendIdentity)' == '' and '%(MauiPlatformSpecificFolder.ActivationProperty)' == '') or ('$(TargetPlatformIdentifier)' != '' and $([System.Text.RegularExpressions.Regex]::Replace(';%(MauiPlatformSpecificFolder.TargetPlatformIdentifiers);', '\s+', '').ToLowerInvariant().Contains(';$(TargetPlatformIdentifier.ToLowerInvariant());'))) or ('$(TargetPlatformIdentifier)' == '' and '%(MauiPlatformSpecificFolder._MauiActivationValueNormalized)' != '' and '%(MauiPlatformSpecificFolder._MauiResolvedActivationValueNormalized)' == '%(MauiPlatformSpecificFolder._MauiActivationValueNormalized)') " />
+    </ItemGroup>
+  </Target>
+
+  <!--
+    Flip ExcludeFromCurrentConfiguration back to false on the kept items so
+    Visual Studio's Solution Explorer / IntelliSense renders them as part of
+    the active configuration. The blanket <Compile Update> at the top of this
+    file unconditionally marks every $(PlatformsProjectFolder)/** file as
+    ExcludeFromCurrentConfiguration=true; the per-TPI flips at the top only
+    cover the built-in $(<TPI>ProjectFolder) paths. Without this target,
+    files in shared folders like Platforms/Apple/ (and neutral-activated
+    backend folders like Platforms/Gtk/) compile correctly but appear
+    grayed-out in the IDE.
+
+    This MUST run AFTER _MauiRemovePlatformCompileItems because that target
+    relies on cross-item-type batching of %(Compile.ExcludeFromCurrentConfiguration);
+    flipping kept items to false beforehand would remove them from the 'true'
+    batch and prevent the to-remove list from being computed correctly.
+  -->
+  <Target Name="_MauiUnflipKeptCompileItemMetadata"
+          AfterTargets="_MauiRemovePlatformCompileItems"
+          Condition=" '$(SingleProject)' == 'true' and '@(_MauiPlatformSpecificCompileItems)' != '' ">
+    <ItemGroup>
+      <!--
+        Dynamic Compile Update item specs apply the metadata to unrelated Compile
+        items. Intersect the current Compile list with the kept platform items,
+        then replace only those matches while preserving their existing metadata.
+      -->
+      <_MauiCompileItemsNotToUnflip Include="@(Compile)" />
+      <_MauiCompileItemsNotToUnflip
+          Remove="@(_MauiPlatformSpecificCompileItems)"
+          MatchOnMetadata="FullPath"
+          MatchOnMetadataOptions="PathLike" />
+      <!--
+        Built-in active-platform items were already flipped to false during
+        evaluation. Skip them here so the remove/re-add below does not perturb
+        Compile ordering and generated WinRT metadata.
+      -->
+      <_MauiCompileItemsToUnflip
+          Include="@(Compile)"
+          Condition=" '%(Compile.ExcludeFromCurrentConfiguration)' == 'true' " />
+      <_MauiCompileItemsToUnflip
+          Remove="@(_MauiCompileItemsNotToUnflip)"
+          MatchOnMetadata="FullPath"
+          MatchOnMetadataOptions="PathLike" />
+      <Compile
+          Remove="@(_MauiCompileItemsToUnflip)"
+          MatchOnMetadata="FullPath"
+          MatchOnMetadataOptions="PathLike" />
+      <Compile Include="@(_MauiCompileItemsToUnflip)">
+        <ExcludeFromCurrentConfiguration>false</ExcludeFromCurrentConfiguration>
+      </Compile>
+    </ItemGroup>
+  </Target>
+
   <!--
     Run before both _MauiInjectXamlCssAdditionalFiles and GenerateMSBuildEditorConfigFileShouldRun because
     if for some reason the _MauiInjectXamlCssAdditionalFiles target is not run, we still get in at the
@@ -46,10 +149,25 @@
 
   <!-- Removals -->
     <ItemGroup>
-      <!-- Remove everything that isn't part of this platform -->
-      <Compile
+      <!--
+        Remove every Compile item under $(PlatformsProjectFolder) that is not
+        part of the current build 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 $(PlatformsProjectFolder)/**
+        file true, then active-platform updates flip matching items back to
+        false. Keep the Condition so only the true batch is removed; otherwise
+        active-platform files may be removed or inactive files may leak in.
+      -->
+      <_MauiPlatformCompileToRemove
           Condition=" '%(Compile.ExcludeFromCurrentConfiguration)' == 'true' "
-          Remove="$(PlatformsProjectFolder)**/*$(DefaultLanguageSourceExtension)" />
+          Include="$(PlatformsProjectFolder)**/*$(DefaultLanguageSourceExtension)"
+          Exclude="@(_MauiPlatformSpecificCompileItems)" />
+      <Compile Remove="@(_MauiPlatformCompileToRemove)" />
 
       <!-- Remove all Windows (WinUI) XAML Files from the Windows folder -->
       <_MauiXamlToRemove