Windows: Detect lack of WS_CLIPCHILDREN when adding child windows

The lack of WS_CLIPCHILDREN will cause drawing artifacts, so ensure
we have WS_CLIPCHILDREN in our native window manual tests, and warn
if users inadvertently end up reparenting windows into a HWND that
doesn't have WS_CLIPCHILDREN set.

Change-Id: Ic4dac83882167562599d63f46232071c8c21b617
Reviewed-by: Zhao Yuhang <2546789017@qq.com>
Reviewed-by: Wladimir Leuschner <wladimir.leuschner@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Pavel Dubsky <pavel.dubsky@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1402,6 +1402,24 @@ void QWindowsBaseWindow::setCustomMargins(const QMargins &)
     Q_UNIMPLEMENTED();
 }
 
+bool QWindowsBaseWindow::windowEvent(QEvent *event)
+{
+    switch (event->type()) {
+    case QEvent::ChildWindowAdded:
+        if (!(GetWindowLongPtr(handle(), GWL_STYLE) & WS_CLIPCHILDREN)) {
+            auto *childWindowEvent = static_cast<QChildWindowEvent*>(event);
+            qWarning() << childWindowEvent->child() << "added as child to"
+                << window() << "which does not have WS_CLIPCHILDREN set."
+                << "This will result in drawing artifacts!";
+        }
+        break;
+    default:
+        break;
+    }
+
+    return QPlatformWindow::windowEvent(event);
+}
+
 /*!
     \class QWindowsDesktopWindow
     \brief Window wrapping GetDesktopWindow not allowing any manipulation.
@@ -2928,7 +2946,7 @@ bool QWindowsWindow::windowEvent(QEvent *event)
         break;
     }
 
-    return QPlatformWindow::windowEvent(event);
+    return QWindowsBaseWindow::windowEvent(event);
 }
 
 void QWindowsWindow::propagateSizeHints()

src/plugins/platforms/windows/qwindowswindow.h

@@ -126,6 +126,8 @@ public:
     static QWindowsBaseWindow *baseWindowOf(const QWindow *w);
     static HWND handleOf(const QWindow *w);
 
+    bool windowEvent(QEvent *event) override;
+
 protected:
     HWND parentHwnd() const { return GetAncestor(handle(), GA_PARENT); }
     bool isTopLevel_sys() const;

tests/manual/embeddedwindows/main.cpp

@@ -93,15 +93,6 @@ int main(int argc, char *argv[])
 
     NativeWindow nativeParentWindow;
     if (QWindow *foreignWindow = QWindow::fromWinId(nativeParentWindow)) {
-
-#ifdef Q_OS_WIN
-        // Native parent windows should have WS_CLIPCHILDREN style set
-        // to prevent overdrawing child area and cause flickering.
-        const HWND hwnd = reinterpret_cast<HWND>(foreignWindow->winId());
-        const LONG_PTR oldStyle = GetWindowLongPtr(hwnd, GWL_STYLE);
-        SetWindowLongPtr(hwnd, GWL_STYLE, oldStyle | WS_CLIPCHILDREN);
-#endif
-
         foreignWindow->setParent(&window);
         foreignWindow->setGeometry(50, 350, 100, 100);
         foreignWindow->showNormal();

tests/shared/nativewindow.h

@@ -133,7 +133,7 @@ NativeWindow::NativeWindow()
         RegisterClass(&wc);
         return wc.lpszClassName;
     }();
-    m_handle = CreateWindowEx(0, className, nullptr, WS_POPUP,
+    m_handle = CreateWindowEx(0, className, nullptr, WS_POPUP | WS_CLIPCHILDREN,
                 CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
                 nullptr, nullptr, GetModuleHandle(nullptr), nullptr);
 }