2 files changed, 80 insertions, 35 deletions

diff --git a/src/widgets/kernel/qdesktopwidget.cpp b/src/widgets/kernel/qdesktopwidget.cpp
index d1b37bc03a..e87b13c630 100644
--- a/src/widgets/kernel/qdesktopwidget.cpp
+++ b/src/widgets/kernel/qdesktopwidget.cpp
@@ -40,6 +40,21 @@
 
 QT_BEGIN_NAMESPACE
 
+QDesktopScreenWidget::QDesktopScreenWidget(QScreen *screen, const QRect &geometry)
+    : QWidget(Q_NULLPTR, Qt::Desktop), m_screen(screen)
+{
+    setVisible(false);
+    if (QWindow *winHandle = windowHandle())
+        winHandle->setScreen(screen);
+    setScreenGeometry(geometry);
+}
+
+void QDesktopScreenWidget::setScreenGeometry(const QRect &geometry)
+{
+    m_geometry = geometry;
+    setGeometry(geometry);
+}
+
 int QDesktopScreenWidget::screenNumber() const
 {
     const QDesktopWidgetPrivate *desktopWidgetP
@@ -74,54 +89,76 @@ const QRect QDesktopWidget::availableGeometry(const QWidget *widget) const
         return rect;
 }
 
+QDesktopScreenWidget *QDesktopWidgetPrivate::widgetForScreen(QScreen *qScreen) const
+{
+    foreach (QDesktopScreenWidget *widget, screens) {
+        if (widget->screen() == qScreen)
+            return widget;
+    }
+    return Q_NULLPTR;
+}
+
 void QDesktopWidgetPrivate::_q_updateScreens()
 {
     Q_Q(QDesktopWidget);
     const QList<QScreen *> screenList = QGuiApplication::screens();
     const int targetLength = screenList.length();
-    const int oldLength = screens.length();
-
-    // Add or remove screen widgets as necessary
-    while (screens.size() > targetLength)
-        delete screens.takeLast();
-
-    for (int currentLength = screens.size(); currentLength < targetLength; ++currentLength) {
-        QScreen *qScreen = screenList.at(currentLength);
-        QDesktopScreenWidget *screenWidget = new QDesktopScreenWidget;
-        screenWidget->setGeometry(qScreen->geometry());
-        QObject::connect(qScreen, SIGNAL(geometryChanged(QRect)),
-                         q, SLOT(_q_updateScreens()), Qt::QueuedConnection);
-        QObject::connect(qScreen, SIGNAL(availableGeometryChanged(QRect)),
-                         q, SLOT(_q_availableGeometryChanged()), Qt::QueuedConnection);
-        QObject::connect(qScreen, SIGNAL(destroyed()),
-                         q, SLOT(_q_updateScreens()), Qt::QueuedConnection);
-        screens.append(screenWidget);
-    }
+    bool screenCountChanged = false;
 
+    // Re-build our screens list. This is the easiest way to later compute which signals to emit.
+    // Create new screen widgets as necessary. While iterating, keep the old list in place so
+    // that widgetForScreen works.
+    // Furthermore, we note which screens have changed, and compute the overall virtual geometry.
+    QList<QDesktopScreenWidget *> newScreens;
+    QList<int> changedScreens;
     QRegion virtualGeometry;
 
-    // update the geometry of each screen widget, determine virtual geometry,
-    // set the new screen for window handle and emit change signals afterwards.
-    QList<int> changedScreens;
-    for (int i = 0; i < screens.length(); i++) {
-        QDesktopScreenWidget *screenWidget = screens.at(i);
+    for (int i = 0; i < targetLength; ++i) {
         QScreen *qScreen = screenList.at(i);
-        QWindow *winHandle = screenWidget->windowHandle();
-        if (winHandle && winHandle->screen() != qScreen)
-            winHandle->setScreen(qScreen);
         const QRect screenGeometry = qScreen->geometry();
-        if (screenGeometry != screenWidget->geometry()) {
-            screenWidget->setGeometry(screenGeometry);
-            changedScreens.push_back(i);
+        QDesktopScreenWidget *screenWidget = widgetForScreen(qScreen);
+        if (screenWidget) {
+            // an old screen. update geometry and remember the index in the *new* list
+            if (screenGeometry != screenWidget->screenGeometry()) {
+                screenWidget->setScreenGeometry(screenGeometry);
+                changedScreens.push_back(i);
+            }
+        } else {
+            // a new screen, create a widget and connect the signals.
+            screenWidget = new QDesktopScreenWidget(qScreen, screenGeometry);
+            QObject::connect(qScreen, SIGNAL(geometryChanged(QRect)),
+                             q, SLOT(_q_updateScreens()), Qt::QueuedConnection);
+            QObject::connect(qScreen, SIGNAL(availableGeometryChanged(QRect)),
+                             q, SLOT(_q_availableGeometryChanged()), Qt::QueuedConnection);
+            QObject::connect(qScreen, SIGNAL(destroyed()),
+                             q, SLOT(_q_updateScreens()), Qt::QueuedConnection);
+            screenCountChanged = true;
         }
+        // record all the screens and the overall geometry.
+        newScreens.push_back(screenWidget);
         virtualGeometry += screenGeometry;
     }
 
+    // Now we apply the accumulated updates.
+    screens.swap(newScreens); // now [newScreens] is the old screen list
+    Q_ASSERT(screens.size() == targetLength);
     q->setGeometry(virtualGeometry.boundingRect());
 
-    if (oldLength != targetLength)
-        emit q->screenCountChanged(targetLength);
+    // Delete the QDesktopScreenWidget that are not used any more.
+    foreach (QDesktopScreenWidget *screen, newScreens) {
+        if (!screens.contains(screen)) {
+            delete screen;
+            screenCountChanged = true;
+        }
+    }
 
+    // Finally, emit the signals.
+    if (screenCountChanged) {
+        // Notice that we trigger screenCountChanged even if a screen was removed and another one added,
+        // in which case the total number of screens did not change. This is the only way for applications
+        // to notice that a screen was swapped out against another one.
+        emit q->screenCountChanged(targetLength);
+    }
     foreach (int changedScreen, changedScreens)
         emit q->resized(changedScreen);
 }
diff --git a/src/widgets/kernel/qdesktopwidget_p.h b/src/widgets/kernel/qdesktopwidget_p.h
index 1fcad7fa65..d5ae629c64 100644
--- a/src/widgets/kernel/qdesktopwidget_p.h
+++ b/src/widgets/kernel/qdesktopwidget_p.h
@@ -55,12 +55,19 @@ QT_BEGIN_NAMESPACE
 class QDesktopScreenWidget : public QWidget {
     Q_OBJECT
 public:
-    QDesktopScreenWidget() : QWidget(Q_NULLPTR, Qt::Desktop)
-    {
-        setVisible(false);
-    }
+    explicit QDesktopScreenWidget(QScreen *screen, const QRect &geometry);
 
     int screenNumber() const;
+    void setScreenGeometry(const QRect &geometry);
+
+    QScreen *screen() const { return m_screen.data(); }
+    QRect screenGeometry() const { return m_geometry; }
+
+private:
+    // The widget updates its screen and geometry automatically. We need to save them separately
+    // to detect changes, and trigger the appropriate signals.
+    const QPointer<QScreen> m_screen;
+    QRect m_geometry;
 };
 
 class QDesktopWidgetPrivate : public QWidgetPrivate {
@@ -70,6 +77,7 @@ public:
     ~QDesktopWidgetPrivate() { qDeleteAll(screens); }
     void _q_updateScreens();
     void _q_availableGeometryChanged();
+    QDesktopScreenWidget *widgetForScreen(QScreen *qScreen) const;
 
     QList<QDesktopScreenWidget *> screens;
 };