Add MileXamlContentWindowDefaultMessageLoop API.

Mile.Xaml.Native/Mile.Xaml.Native.def

@@ -5,6 +5,7 @@ EXPORTS
 MileXamlGetCoreWindowHandle
 MileXamlSetXamlContentForContentWindow
 MileXamlContentWindowDefaultCallback
+MileXamlContentWindowDefaultMessageLoop
 MileXamlGetTransparentBackgroundAttribute
 MileXamlSetTransparentBackgroundAttribute
 MileXamlGetPreferredDarkModeAttribute

Mile.Xaml.Native/Mile.Xaml.cpp

@@ -412,6 +412,31 @@ EXTERN_C LRESULT CALLBACK MileXamlContentWindowDefaultCallback(
     return 0;
 }
 
+EXTERN_C int WINAPI MileXamlContentWindowDefaultMessageLoop()
+{
+    MSG Message;
+    while (::GetMessageW(&Message, nullptr, 0, 0))
+    {
+        // Workaround for capturing Alt+F4 in applications with XAML Islands.
+        // Reference: https://github.com/microsoft/microsoft-ui-xaml/issues/2408
+        if (Message.message == WM_SYSKEYDOWN && Message.wParam == VK_F4)
+        {
+            ::SendMessageW(
+                ::GetAncestor(Message.hwnd, GA_ROOT),
+                Message.message,
+                Message.wParam,
+                Message.lParam);
+
+            continue;
+        }
+
+        ::TranslateMessage(&Message);
+        ::DispatchMessageW(&Message);
+    }
+
+    return static_cast<int>(Message.wParam);
+}
+
 EXTERN_C HRESULT WINAPI MileXamlGetTransparentBackgroundAttribute(
     _Out_ PBOOLEAN TransparentBackground)
 {

Mile.Xaml.Native/Mile.Xaml.h

@@ -52,6 +52,13 @@ EXTERN_C LRESULT CALLBACK MileXamlContentWindowDefaultCallback(
     _In_ WPARAM wParam,
     _In_ LPARAM lParam);
 
+/**
+ * @brief The default message loop function for Mile.Xaml.ContentWindow
+ *        window class.
+ * @return The message loop exit code.
+ */
+EXTERN_C int WINAPI MileXamlContentWindowDefaultMessageLoop();
+
 /**
  * @brief Retrieves the transparent background attribute for contents hosted
  *        via XAML Islands.