Windows: Ensure clipboard is flushed out before QGuiApplication is destroyed.

Otherwise, OleFlushClipboard() might query the data again which causes
problems for QMimeData-derived classes using QPixmap/QImage.

Task-number: QTBUG-36958

Change-Id: I89e58eeb64bd3481e89ad789f310f19ddb4604a2
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

src/plugins/platforms/windows/qwindowsclipboard.cpp

@@ -132,19 +132,35 @@ extern "C" LRESULT QT_WIN_CALLBACK qClipboardViewerWndProc(HWND hwnd, UINT messa
     return DefWindowProc(hwnd, message, wParam, lParam);
 }
 
+// QTBUG-36958, ensure the clipboard is flushed before
+// QGuiApplication is destroyed since OleFlushClipboard()
+// might query the data again which causes problems
+// for QMimeData-derived classes using QPixmap/QImage.
+static void cleanClipboardPostRoutine()
+{
+    if (QWindowsClipboard *cl = QWindowsClipboard::instance())
+        cl->cleanup();
+}
+
 QWindowsClipboard *QWindowsClipboard::m_instance = 0;
 
 QWindowsClipboard::QWindowsClipboard() :
     m_data(0), m_clipboardViewer(0), m_nextClipboardViewer(0)
 {
     QWindowsClipboard::m_instance = this;
+    qAddPostRoutine(cleanClipboardPostRoutine);
 }
 
 QWindowsClipboard::~QWindowsClipboard()
+{
+    cleanup();
+    QWindowsClipboard::m_instance = 0;
+}
+
+void QWindowsClipboard::cleanup()
 {
     unregisterViewer(); // Should release data if owner.
     releaseIData();
-    QWindowsClipboard::m_instance = 0;
 }
 
 void QWindowsClipboard::releaseIData()

src/plugins/platforms/windows/qwindowsclipboard.h

@@ -64,6 +64,7 @@ public:
     QWindowsClipboard();
     ~QWindowsClipboard();
     void registerViewer(); // Call in initialization, when context is up.
+    void cleanup();
 
     virtual QMimeData *mimeData(QClipboard::Mode mode = QClipboard::Clipboard);
     virtual void setMimeData(QMimeData *data, QClipboard::Mode mode = QClipboard::Clipboard);