Explicitly measure relative to the root XAML element (#10665)

* Explicitly measure relative to the root XAML element

Previously, `measureInWindow` measured relative to
`xaml::Window::Current().Content()`, which is null for XAML Islands
applications. Calling `TransformToVisual` with a null argument measures
relative to the root XAML content, which is equivalent to
`xaml::Window::Current().Content()` for the foreground window. The main
difference is that this API will now measure correctly for React Native
windows that are not in the foreground.

* Change files

change/react-native-windows-0f54d4e2-d544-4e6b-b6bb-3f7e6eb6d3e3.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Explicitly measure relative to the root XAML element",
+  "packageName": "react-native-windows",
+  "email": "erozell@outlook.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Modules/NativeUIManager.cpp

@@ -1012,7 +1012,9 @@ void NativeUIManager::measureInWindow(
   ShadowNodeBase &node = static_cast<ShadowNodeBase &>(shadowNode);
 
   if (auto view = node.GetView().try_as<xaml::FrameworkElement>()) {
-    auto windowTransform = view.TransformToVisual(xaml::Window::Current().Content());
+    // When supplied with nullptr, TransformToVisual will return the position
+    // relative to the root XAML element.
+    auto windowTransform = view.TransformToVisual(nullptr);
     auto positionInWindow = windowTransform.TransformPoint({0, 0});
 
     m_context.JSDispatcher().Post(