WIP

src/CMakeLists.txt

@@ -154,6 +154,10 @@ if (NOT WIN32 AND NOT APPLE)
     target_link_libraries(kddockwidgets PUBLIC Qt5::X11Extras)
 endif()
 
+if (WIN32)
+    target_link_libraries(kddockwidgets PUBLIC Qt5::GuiPrivate)
+endif()
+
 install (TARGETS kddockwidgets kddockwidgets_multisplitter
          EXPORT kddockwidgetsTargets
          RUNTIME DESTINATION bin

src/private/Utils_p.h

@@ -32,6 +32,14 @@
 # include <QtX11Extras/QX11Info>
 #endif
 
+#ifdef Q_OS_WIN
+# include <QMargins>
+# include <QWindow>
+# include <QVariant>
+# include <qpa/qplatformnativeinterface.h>
+# include <Windows.h>
+#endif
+
 namespace KDDockWidgets {
 
 inline bool isLeftButtonPressed()
@@ -92,6 +100,33 @@ inline int screenNumberForWidget(const QWidget *w)
     return -1;
 }
 
-};
+inline void extendClientAreaOverTitleBar(QWindow *window)
+{
+    if (!window)
+        return;
+#ifdef Q_OS_WIN
+    QPlatformWindow *platformWindow = window->handle();
+    if (!platformWindow)
+        return;
+
+    auto hwnd = HWND(window->winId());
+    const auto style = GetWindowLongPtr(hwnd, GWL_STYLE);
+    const auto exStyle = GetWindowLongPtr(hwnd, GWL_EXSTYLE);
+
+    const RECT arbitraryClientRect = { 0, 0, 800, 800 };
+    RECT ncRect = arbitraryClientRect;  
+
+    AdjustWindowRectEx(&ncRect, style, FALSE, exStyle);
+    const int titleBarHeight = arbitraryClientRect.top - ncRect.top;
+
+    const QVariant newMargins = QVariant::fromValue(QMargins(0, -titleBarHeight, 0, 0));
+    window->setProperty("_q_windowsCustomMargins", newMargins);
+    QGuiApplication::platformNativeInterface()->setWindowProperty(platformWindow, QStringLiteral("WindowsCustomMargins"), newMargins);
+#endif
+}
+
+}
+
+Q_DECLARE_METATYPE(QMargins);
 
 #endif

src/private/WidgetResizeHandler.cpp

@@ -215,8 +215,8 @@ bool WidgetResizeHandler::handleWindowsNativeEvent(FloatingWindow *w, const QByt
 
     auto msg = static_cast<MSG *>(message);
     if (msg->message == WM_NCCALCSIZE) {
-        *result = 0;
+        // Let's use Qt's "WindowCustomMargins" instead, otherwise QWindow won't know
-        return true;
+        return false;
     } else if (msg->message == WM_NCHITTEST) {
 
         if (DragController::instance()->isInClientDrag()) {

src/private/widgets/FloatingWindowWidget.cpp

@@ -23,7 +23,6 @@
 #include "Utils_p.h"
 #include "DropArea_p.h"
 #include "TitleBar_p.h"
-
 #include <QApplication>
 #include <QPainter>
 #include <QVBoxLayout>
@@ -60,6 +59,21 @@ bool FloatingWindowWidget::event(QEvent *ev)
     return FloatingWindow::event(ev);
 }
 
+#ifdef Q_OS_WIN
+bool FloatingWindowWidget::nativeEvent(const QByteArray &eventType, void *message, long *result)
+{
+    if (!KDDockWidgets::usesAeroSnapWithCustomDecos() || eventType != "windows_generic_MSG")
+        return QWidget::nativeEvent(eventType, message, result);
+
+    auto m = static_cast<MSG*>(message);
+    if (m->message != WM_DPICHANGED)
+        return QWidget::nativeEvent(eventType, message, result);
+
+    KDDockWidgets::extendClientAreaOverTitleBar(windowHandle());
+    return QWidget::nativeEvent(eventType, message, result);
+}
+#endif
+
 void FloatingWindowWidget::init()
 {
     m_vlayout->setSpacing(0);
@@ -69,4 +83,7 @@ void FloatingWindowWidget::init()
 
     if (!KDDockWidgets::usesNativeDraggingAndResizing())
         m_vlayout->setContentsMargins(4, 4, 4, 4);
+
+    if (KDDockWidgets::usesAeroSnapWithCustomDecos())
+        KDDockWidgets::extendClientAreaOverTitleBar(windowHandle());
 }

src/private/widgets/FloatingWindowWidget_p.h

@@ -39,6 +39,10 @@ public:
 protected:
     void paintEvent(QPaintEvent *) override;
     bool event(QEvent *ev) override;
+#ifdef Q_OS_WIN
+    bool nativeEvent(const QByteArray &eventType, void *message, long *result) override;
+#endif
+
 private:
     void init();
     Q_DISABLE_COPY(FloatingWindowWidget)