Creating an instance of the QDesktopWidget breaks clipboard on X11.

On X11 when user manually creates a QDesktopWidget object it breaks
notifications from the root window since we have several
QDesktopWidgets that have the same native window id and the
wid->qwidget mapper (QWidgetPrivate::mapper) goes crazy. So whenever
we get x11 event from the root window we cannot find a qwidget that
corresponds to the window id and discard these events.

Reviewed-by: Brad

2 files changed, 17 insertions, 3 deletions

diff --git a/doc/src/qdesktopwidget.qdoc b/doc/src/qdesktopwidget.qdoc
index 0361aae..5a27fb4 100644
--- a/doc/src/qdesktopwidget.qdoc
+++ b/doc/src/qdesktopwidget.qdoc
@@ -48,6 +48,9 @@
     \ingroup environment
     \mainclass
 
+    QApplication::desktop() function should be used to get an instance
+    of the QDesktopWidget.
+
     Systems with more than one graphics card and monitor can manage the
     physical screen space available either as multiple desktops, or as a
     large virtual desktop, which usually has the size of the bounding
@@ -86,12 +89,14 @@
     screens. The correct width and height values are obtained using
     availableGeometry() or screenGeometry() for a particular screen.
 
-    \sa QApplication, QX11Info::appRootWindow()
+    \sa QApplication, QApplication::desktop(), QX11Info::appRootWindow()
 */
 
 /*!
     \fn QDesktopWidget::QDesktopWidget()
 
+    \internal
+
     Creates the desktop widget.
 
     If the system supports a virtual desktop, this widget will have
@@ -104,6 +109,8 @@
 /*!
     \fn QDesktopWidget::~QDesktopWidget()
 
+    \internal
+
     Destroys the desktop widget and frees any allocated resources.
 */
 
diff --git a/src/gui/kernel/qwidget.cpp b/src/gui/kernel/qwidget.cpp
index 06810e0..dcb48cc 100644
--- a/src/gui/kernel/qwidget.cpp
+++ b/src/gui/kernel/qwidget.cpp
@@ -164,6 +164,7 @@ static inline bool bypassGraphicsProxyWidget(QWidget *p)
 #endif
 
 extern bool qt_sendSpontaneousEvent(QObject*, QEvent*); // qapplication.cpp
+extern QDesktopWidget *qt_desktopWidget; // qapplication.cpp
 
 QWidgetPrivate::QWidgetPrivate(int version) :
         QObjectPrivate(version), extra(0), focus_child(0)
@@ -1400,7 +1401,13 @@ int QWidgetPrivate::maxInstances = 0;     // Maximum number of widget instances
 void QWidgetPrivate::setWinId(WId id)                // set widget identifier
 {
     Q_Q(QWidget);
-    if (mapper && data.winid) {
+    // the user might create a widget with Qt::Desktop window
+    // attribute (or create another QDesktopWidget instance), which
+    // will have the same windowid (the root window id) as the
+    // qt_desktopWidget. We should not add the second desktop widget
+    // to the mapper.
+    bool userDesktopWidget = qt_desktopWidget != 0 && qt_desktopWidget != q && q->windowType() == Qt::Desktop;
+    if (mapper && data.winid && !userDesktopWidget) {
         mapper->remove(data.winid);
         uncreatedWidgets->insert(q);
     }
@@ -1409,7 +1416,7 @@ void QWidgetPrivate::setWinId(WId id)                // set widget identifier
 #if defined(Q_WS_X11)
     hd = id; // X11: hd == ident
 #endif
-    if (mapper && id) {
+    if (mapper && id && !userDesktopWidget) {
         mapper->insert(data.winid, q);
         uncreatedWidgets->remove(q);
     }