Improve handling of screens in QWidgets.

Historically, the screen number of a QWidget was stored in
QTLWExtra::screenIndex which came in via the Q_WS_QWS platform.
The variable contained the screen number of the QDesktopScreenWidget
and the desired screen number for widget creation (from parent widget
or desktop parent widgets) and was not updated according to widget
geometry or when the associated QWindow moved to another screen.

Storing the screen number is problematic in Qt 5 since it may change
when the list of QScreens in QGuiApplication is rearranged.

This change renames it to initialScreen and changes its usage to only
contain a screen number when a screen is requested by parenting
on a desktop widget or reparenting. A value of -1 indicates no screen
is requested and the number should be deduced from the geometry.

For the usage in QDesktopScreenWidget, add a method to determine the
number via index in the list of QDesktopWidget.

Task-number: QTBUG-44070
Task-number: QTBUG-44213
Change-Id: I153d19073ad4b0fd14ef3d24caa6d3e76f350e82
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/widgets/kernel/qdesktopwidget.cpp

@@ -39,6 +39,13 @@
 
 QT_BEGIN_NAMESPACE
 
+int QDesktopScreenWidget::screenNumber() const
+{
+    const QDesktopWidgetPrivate *desktopWidgetP
+        = static_cast<const QDesktopWidgetPrivate *>(qt_widget_private(QApplication::desktop()));
+    return desktopWidgetP->screens.indexOf(const_cast<QDesktopScreenWidget *>(this));
+}
+
 const QRect QDesktopWidget::screenGeometry(const QWidget *widget) const
 {
     if (!widget) {
@@ -79,7 +86,7 @@ void QDesktopWidgetPrivate::_q_updateScreens()
 
     for (int currentLength = screens.size(); currentLength < targetLength; ++currentLength) {
         QScreen *qScreen = screenList.at(currentLength);
-        QDesktopScreenWidget *screenWidget = new QDesktopScreenWidget(currentLength);
+        QDesktopScreenWidget *screenWidget = new QDesktopScreenWidget;
         screenWidget->setGeometry(qScreen->geometry());
         QObject::connect(qScreen, SIGNAL(geometryChanged(QRect)),
                          q, SLOT(_q_updateScreens()), Qt::QueuedConnection);

src/widgets/kernel/qdesktopwidget_p.h

@@ -53,12 +53,12 @@ QT_BEGIN_NAMESPACE
 class QDesktopScreenWidget : public QWidget {
     Q_OBJECT
 public:
-    QDesktopScreenWidget(int screenNumber = -1) : QWidget(0, Qt::Desktop)
+    QDesktopScreenWidget() : QWidget(Q_NULLPTR, Qt::Desktop)
     {
         setVisible(false);
-        QTLWExtra *topData = d_func()->topData();
-        topData->screenIndex = screenNumber;
     }
+
+    int screenNumber() const;
 };
 
 class QDesktopWidgetPrivate : public QWidgetPrivate {

src/widgets/kernel/qwidget.cpp

@@ -35,7 +35,7 @@
 #include "qapplication_p.h"
 #include "qbrush.h"
 #include "qcursor.h"
-#include "qdesktopwidget.h"
+#include "qdesktopwidget_p.h"
 #include "qevent.h"
 #include "qhash.h"
 #include "qlayout.h"
@@ -1111,9 +1111,10 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
     if (allWidgets)
         allWidgets->insert(q);
 
-    QWidget *desktopWidget = 0;
+    int targetScreen = -1;
     if (parentWidget && parentWidget->windowType() == Qt::Desktop) {
-        desktopWidget = parentWidget;
+        const QDesktopScreenWidget *sw = qobject_cast<const QDesktopScreenWidget *>(parentWidget);
+        targetScreen = sw ? sw->screenNumber() : 0;
         parentWidget = 0;
     }
 
@@ -1133,10 +1134,10 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
         xinfo = desktopWidget->d_func()->xinfo;
     }
 #endif
-    if (desktopWidget) {
-        const int screen = desktopWidget->d_func()->topData()->screenIndex;
+    if (targetScreen >= 0) {
+        topData()->initialScreenIndex = targetScreen;
         if (QWindow *window = q->windowHandle())
-            window->setScreen(QGuiApplication::screens().value(screen, 0));
+            window->setScreen(QGuiApplication::screens().value(targetScreen, Q_NULLPTR));
     }
 
     data.fstrut_dirty = true;
@@ -1414,8 +1415,15 @@ void QWidgetPrivate::create_sys(WId window, bool initializeWindow, bool destroyO
         win->setGeometry(q->geometry());
     else
         win->resize(q->size());
-    if (win->isTopLevel())
-        win->setScreen(QGuiApplication::screens().value(topData()->screenIndex, 0));
+    if (win->isTopLevel()) {
+        int screenNumber = topData()->initialScreenIndex;
+        topData()->initialScreenIndex = -1;
+        if (screenNumber < 0) {
+            screenNumber = q->windowType() != Qt::Desktop
+                ? QApplication::desktop()->screenNumber(q) : 0;
+        }
+        win->setScreen(QGuiApplication::screens().value(screenNumber, Q_NULLPTR));
+    }
 
     QSurfaceFormat format = win->requestedFormat();
     if ((flags & Qt::Window) && win->surfaceType() != QSurface::OpenGLSurface
@@ -1715,7 +1723,7 @@ void QWidgetPrivate::createTLExtra()
         x->embedded = 0;
         x->window = 0;
         x->shareContext = 0;
-        x->screenIndex = 0;
+        x->initialScreenIndex = -1;
 #ifdef Q_WS_MAC
         x->wasMaximized = false;
 #endif // Q_WS_MAC
@@ -10503,9 +10511,8 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
     if (newparent && newparent->windowType() == Qt::Desktop) {
         // make sure the widget is created on the same screen as the
         // programmer specified desktop widget
-
-        // get the desktop's screen number
-        targetScreen = newparent->window()->d_func()->topData()->screenIndex;
+        const QDesktopScreenWidget *sw = qobject_cast<const QDesktopScreenWidget *>(newparent);
+        targetScreen = sw ? sw->screenNumber() : 0;
         newparent = 0;
     }
 
@@ -10537,7 +10544,7 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
         f |= Qt::Window;
         if (targetScreen == -1) {
             if (parent)
-                targetScreen = q->parentWidget()->window()->d_func()->topData()->screenIndex;
+                targetScreen = QApplication::desktop()->screenNumber(q->parentWidget()->window());
         }
     }
 
@@ -10581,12 +10588,11 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
 
     // move the window to the selected screen
     if (!newparent && targetScreen != -1) {
-        if (maybeTopData())
-            maybeTopData()->screenIndex = targetScreen;
         // only if it is already created
-        if (q->testAttribute(Qt::WA_WState_Created)) {
+        if (q->testAttribute(Qt::WA_WState_Created))
             q->windowHandle()->setScreen(QGuiApplication::screens().value(targetScreen, 0));
-        }
+        else
+            topData()->initialScreenIndex = targetScreen;
     }
 }
 

src/widgets/kernel/qwidget_p.h

@@ -212,7 +212,7 @@ struct QTLWExtra {
 #endif
     QWidgetWindow *window;
     QOpenGLContext *shareContext;
-    quint32 screenIndex; // index in qplatformscreenlist
+    int initialScreenIndex; // Screen number when passing a QDesktop[Screen]Widget as parent.
 };
 
 struct QWExtra {