refactor(new_editor/app): isolate cross-window drop support

new_editor/app/Platform/Win32/EditorWindowManagerLookup.cpp

@@ -54,4 +54,45 @@ const EditorWindow* EditorWindowManager::FindPrimaryWindow() const {
     return const_cast<EditorWindowManager*>(this)->FindPrimaryWindow();
 }
 
+EditorWindow* EditorWindowManager::FindTopmostWindowAtScreenPoint(
+    const POINT& screenPoint,
+    std::string_view excludedWindowId) {
+    if (const HWND hitWindow = WindowFromPoint(screenPoint); hitWindow != nullptr) {
+        const HWND rootWindow = GetAncestor(hitWindow, GA_ROOT);
+        if (EditorWindow* window = FindWindow(rootWindow);
+            window != nullptr &&
+            window->GetWindowId() != excludedWindowId) {
+            return window;
+        }
+    }
+
+    for (auto it = m_windows.rbegin(); it != m_windows.rend(); ++it) {
+        EditorWindow* const window = it->get();
+        if (window == nullptr ||
+            window->GetHwnd() == nullptr ||
+            window->GetWindowId() == excludedWindowId) {
+            continue;
+        }
+
+        RECT windowRect = {};
+        if (GetWindowRect(window->GetHwnd(), &windowRect) &&
+            screenPoint.x >= windowRect.left &&
+            screenPoint.x < windowRect.right &&
+            screenPoint.y >= windowRect.top &&
+            screenPoint.y < windowRect.bottom) {
+            return window;
+        }
+    }
+
+    return nullptr;
+}
+
+const EditorWindow* EditorWindowManager::FindTopmostWindowAtScreenPoint(
+    const POINT& screenPoint,
+    std::string_view excludedWindowId) const {
+    return const_cast<EditorWindowManager*>(this)->FindTopmostWindowAtScreenPoint(
+        screenPoint,
+        excludedWindowId);
+}
+
 } // namespace XCEngine::UI::Editor::App