Enable DWM transparency for OpenGL windows under Windows

Starting with Windows Vista, we can control the compositing window
manager using the DWM APIs. This allows us to make truly transparent
OpenGL windows.

We also have to detect and listen if compositing is enabled, otherwise
enabling transparency will cause glitches all over the place.

This (partially) fixes
Task-number: QTBUG-28214

Change-Id: I0fe1ec7adec8181b788c32de03c59142731d9e7f
Done-with: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

5 files changed, 86 insertions, 0 deletions

diff --git a/src/plugins/platforms/windows/qtwindows_additional.h b/src/plugins/platforms/windows/qtwindows_additional.h
index 49ddf3106b..4c08a664d8 100644
--- a/src/plugins/platforms/windows/qtwindows_additional.h
+++ b/src/plugins/platforms/windows/qtwindows_additional.h
@@ -49,6 +49,10 @@
 #    define WM_THEMECHANGED 0x031A
 #endif
 
+#ifndef WM_DWMCOMPOSITIONCHANGED
+#    define WM_DWMCOMPOSITIONCHANGED 0x31E
+#endif
+
 #ifndef GWL_HWNDPARENT
 #    define GWL_HWNDPARENT (-8)
 #endif
diff --git a/src/plugins/platforms/windows/qtwindowsglobal.h b/src/plugins/platforms/windows/qtwindowsglobal.h
index 64bf1d71ca..e9eb50799e 100644
--- a/src/plugins/platforms/windows/qtwindowsglobal.h
+++ b/src/plugins/platforms/windows/qtwindowsglobal.h
@@ -105,6 +105,7 @@ enum WindowsEventType // Simplify event types
     InputMethodCloseCandidateWindowEvent = InputMethodEventFlag + 5,
     InputMethodRequest = InputMethodEventFlag + 6,
     ThemeChanged = ThemingEventFlag + 1,
+    CompositionSettingsChanged = ThemingEventFlag + 2,
     DisplayChangedEvent = 437,
     SettingChangedEvent = DisplayChangedEvent + 1,
     ContextMenu = 123,
@@ -201,6 +202,8 @@ inline QtWindows::WindowsEventType windowsEventType(UINT message, WPARAM wParamI
         return QtWindows::DisplayChangedEvent;
     case WM_THEMECHANGED:
         return QtWindows::ThemeChanged;
+    case WM_DWMCOMPOSITIONCHANGED:
+        return QtWindows::CompositionSettingsChanged;
 #ifndef QT_NO_CONTEXTMENU
     case WM_CONTEXTMENU:
         return QtWindows::ContextMenu;
diff --git a/src/plugins/platforms/windows/qwindowscontext.cpp b/src/plugins/platforms/windows/qwindowscontext.cpp
index 132b3684e9..c676ca3c46 100644
--- a/src/plugins/platforms/windows/qwindowscontext.cpp
+++ b/src/plugins/platforms/windows/qwindowscontext.cpp
@@ -884,6 +884,9 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
             theme->windowsThemeChanged(platformWindow->window());
         return true;
     }
+    case QtWindows::CompositionSettingsChanged:
+        platformWindow->handleCompositionSettingsChanged();
+        return true;
 #ifndef Q_OS_WINCE
     case QtWindows::ActivateWindowEvent:
 #ifndef QT_NO_TABLETEVENT
diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index 8ec10294a2..b3b07cc8dd 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -56,6 +56,7 @@
 #include <QtGui/QScreen>
 #include <QtGui/QWindow>
 #include <QtGui/QRegion>
+#include <private/qsystemlibrary_p.h>
 #include <private/qwindow_p.h>
 #include <private/qguiapplication_p.h>
 #include <qpa/qwindowsysteminterface.h>
@@ -203,6 +204,69 @@ static inline QSize clientSize(HWND hwnd)
     return qSizeOfRect(rect);
 }
 
+static bool applyBlurBehindWindow(HWND hwnd)
+{
+#ifdef Q_OS_WINCE
+    Q_UNUSED(hwnd);
+    return false;
+#else
+    enum { dwmBbEnable = 0x1, dwmBbBlurRegion = 0x2 };
+
+    struct DwmBlurBehind {
+        DWORD dwFlags;
+        BOOL  fEnable;
+        HRGN  hRgnBlur;
+        BOOL  fTransitionOnMaximized;
+    };
+
+    typedef HRESULT (WINAPI *PtrDwmEnableBlurBehindWindow)(HWND, const DwmBlurBehind*);
+    typedef HRESULT (WINAPI *PtrDwmIsCompositionEnabled)(BOOL *);
+
+    // DWM API is available only from Windows Vista
+    if (QSysInfo::windowsVersion() < QSysInfo::WV_VISTA)
+        return false;
+
+    static bool functionPointersResolved = false;
+    static PtrDwmEnableBlurBehindWindow dwmBlurBehind = 0;
+    static PtrDwmIsCompositionEnabled dwmIsCompositionEnabled = 0;
+
+    if (Q_UNLIKELY(!functionPointersResolved)) {
+        QSystemLibrary library(QStringLiteral("dwmapi"));
+        if (library.load()) {
+            dwmBlurBehind = (PtrDwmEnableBlurBehindWindow)(library.resolve("DwmEnableBlurBehindWindow"));
+            dwmIsCompositionEnabled = (PtrDwmIsCompositionEnabled)(library.resolve("DwmIsCompositionEnabled"));
+        }
+
+        functionPointersResolved = true;
+    }
+
+    if (Q_UNLIKELY(!dwmBlurBehind || !dwmIsCompositionEnabled))
+        return false;
+
+    BOOL compositionEnabled;
+    if (dwmIsCompositionEnabled(&compositionEnabled) != S_OK)
+        return false;
+
+    DwmBlurBehind blurBehind = {0, 0, 0, 0};
+
+    if (compositionEnabled) {
+        blurBehind.dwFlags = dwmBbEnable | dwmBbBlurRegion;
+        blurBehind.fEnable = TRUE;
+        blurBehind.hRgnBlur = CreateRectRgn(0, 0, -1, -1);
+    } else {
+        blurBehind.dwFlags = dwmBbEnable;
+        blurBehind.fEnable = FALSE;
+    }
+
+    const bool result = dwmBlurBehind(hwnd, &blurBehind) == S_OK;
+
+    if (blurBehind.hRgnBlur)
+        DeleteObject(blurBehind.hRgnBlur);
+
+    return result;
+#endif // Q_OS_WINCE
+}
+
 // from qwidget_win.cpp, pass flags separately in case they have been "autofixed".
 static bool shouldShowMaximizeButton(const QWindow *w, Qt::WindowFlags flags)
 {
@@ -531,6 +595,10 @@ QWindowsWindow::WindowData
     result.frame = context->margins;
     result.embedded = embedded;
     result.customMargins = context->customMargins;
+
+    if (isGL && hasAlpha)
+        applyBlurBehindWindow(result.hwnd);
+
     return result;
 }
 
@@ -1155,6 +1223,13 @@ void QWindowsWindow::handleHidden()
     fireExpose(QRegion());
 }
 
+void QWindowsWindow::handleCompositionSettingsChanged()
+{
+    const QWindow *w = window();
+    if (w->surfaceType() == QWindow::OpenGLSurface && w->format().hasAlpha())
+        applyBlurBehindWindow(handle());
+}
+
 void QWindowsWindow::setGeometry(const QRect &rectIn)
 {
     QRect rect = rectIn;
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index f7d142fc36..d365ac4d97 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/src/plugins/platforms/windows/qwindowswindow.h
@@ -221,6 +221,7 @@ public:
     void handleMoved();
     void handleResized(int wParam);
     void handleHidden();
+    void handleCompositionSettingsChanged();
 
     static inline HWND handleOf(const QWindow *w);
     static inline QWindowsWindow *baseWindowOf(const QWindow *w);