Revert "Windows QPA: Remove usage of deprecated DwmEnableBlurBehindWindow"

This reverts commit b07876105788d519a3bb04a353eaa5ffa6269253.

Reason for revert: Introduced regression QTBUG-136098

Change-Id: Id1f4d7a485659c4150f6750866c2f702fcdc1bf6
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1233,6 +1233,7 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
         return true;
     }
     case QtWindows::CompositionSettingsChanged:
+        platformWindow->handleCompositionSettingsChanged();
         return true;
     case QtWindows::ActivateWindowEvent:
         if (platformWindow->window()->flags() & Qt::WindowDoesNotAcceptFocus) {

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -485,6 +485,22 @@ static inline bool windowIsAccelerated(const QWindow *w)
     }
 }
 
+static bool applyBlurBehindWindow(HWND hwnd)
+{
+    DWM_BLURBEHIND blurBehind = {0, 0, nullptr, 0};
+
+    blurBehind.dwFlags = DWM_BB_ENABLE | DWM_BB_BLURREGION;
+    blurBehind.fEnable = TRUE;
+    blurBehind.hRgnBlur = CreateRectRgn(0, 0, -1, -1);
+
+    const bool result = DwmEnableBlurBehindWindow(hwnd, &blurBehind) == S_OK;
+
+    if (blurBehind.hRgnBlur)
+        DeleteObject(blurBehind.hRgnBlur);
+
+    return result;
+}
+
 // from qwidget_win.cpp, pass flags separately in case they have been "autofixed".
 static bool shouldShowMaximizeButton(const QWindow *w, Qt::WindowFlags flags)
 {
@@ -1044,6 +1060,8 @@ void WindowCreationData::initialize(const QWindow *w, HWND hwnd, bool frameChang
 
     const bool isAccelerated = windowIsAccelerated(w);
     const bool hasAlpha = w->format().hasAlpha();
+    if (isAccelerated && hasAlpha)
+        applyBlurBehindWindow(hwnd);
     setWindowOpacity(hwnd, flags, hasAlpha, isAccelerated, opacityLevel);
 }
 
@@ -2096,6 +2114,13 @@ void QWindowsWindow::handleHidden()
     fireExpose(QRegion());
 }
 
+void QWindowsWindow::handleCompositionSettingsChanged()
+{
+    const QWindow *w = window();
+    if (windowIsAccelerated(w) && w->format().hasAlpha())
+        applyBlurBehindWindow(handle());
+}
+
 qreal QWindowsWindow::dpiRelativeScale(const UINT dpi) const
 {
     return QHighDpiScaling::roundScaleFactor(qreal(dpi) / QWindowsScreen::baseDpi) /

src/plugins/platforms/windows/qwindowswindow.h

@@ -295,6 +295,7 @@ public:
     void handleMoved();
     void handleResized(int wParam, LPARAM lParam);
     void handleHidden();
+    void handleCompositionSettingsChanged();
     void handleDpiScaledSize(WPARAM wParam, LPARAM lParam, LRESULT *result);
     void handleDpiChanged(HWND hwnd, WPARAM wParam, LPARAM lParam);
     void handleDpiChangedAfterParent(HWND hwnd);