Add WindowProc delegation to Windows template (#64166)

This wires up the new WindowProc delegation system that allows plugins
to handle top-level window messages (e.g., to control resize behavior).

Fixes #53168

packages/flutter_tools/templates/app/windows.tmpl/runner/flutter_window.cpp

 #include "flutter_window.h"
 
+#include <optional>
+
 #include "flutter/generated_plugin_registrant.h"
 
 FlutterWindow::FlutterWindow(RunLoop* run_loop,
@@ -34,3 +36,19 @@ void FlutterWindow::OnDestroy() {
 
   Win32Window::OnDestroy();
 }
+
+LRESULT
+FlutterWindow::MessageHandler(HWND hwnd, UINT const message,
+                              WPARAM const wparam,
+                              LPARAM const lparam) noexcept {
+  // Give Flutter, including plugins, an opporutunity to handle window messages.
+  if (flutter_controller_) {
+    std::optional<LRESULT> result =
+        flutter_controller_->HandleTopLevelWindowProc(hwnd, message, wparam,
+                                                      lparam);
+    if (result) {
+      return *result;
+    }
+  }
+  return Win32Window::MessageHandler(hwnd, message, wparam, lparam);
+}

packages/flutter_tools/templates/app/windows.tmpl/runner/flutter_window.h

@@ -4,11 +4,11 @@
 #include <flutter/dart_project.h>
 #include <flutter/flutter_view_controller.h>
 
+#include <memory>
+
 #include "run_loop.h"
 #include "win32_window.h"
 
-#include <memory>
-
 // A window that does nothing but host a Flutter view.
 class FlutterWindow : public Win32Window {
  public:
@@ -22,6 +22,8 @@ class FlutterWindow : public Win32Window {
   // Win32Window:
   bool OnCreate() override;
   void OnDestroy() override;
+  LRESULT MessageHandler(HWND window, UINT const message, WPARAM const wparam,
+                         LPARAM const lparam) noexcept override;
 
  private:
   // The run loop driving events for this window.

packages/flutter_tools/templates/app/windows.tmpl/runner/win32_window.cpp

@@ -154,13 +154,6 @@ Win32Window::MessageHandler(HWND hwnd,
                             UINT const message,
                             WPARAM const wparam,
                             LPARAM const lparam) noexcept {
-  auto window =
-      reinterpret_cast<Win32Window*>(GetWindowLongPtr(hwnd, GWLP_USERDATA));
-
-  if (window == nullptr) {
-    return 0;
-  }
-
   switch (message) {
     case WM_DESTROY:
       window_handle_ = nullptr;