Introduce QWidget::setScreen

Follows the QWindow semantics, and is a replacement for creating
a QWidget with a QDesktopScreenWidget as the parent.

We can now remove much of the special handling of QDesktopWidget and
the Qt::Desktop window type, and get rid of QDesktopScreenWidget.

Add a manual test that allows local testing. Our CI environments
only have a single screen, and no multi-head display server setup
which is the primary case where QWidget::setScreen is interesting.
For the more common case of a virtual desktop, QWidget::setScreen
has no real impact (just as QWindow::setScreen doesn't).

Change-Id: Id0099e069d316741bacd8c795c396ccad37be297
Fixes: QTBUG-85483
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

14 files changed, 239 insertions, 64 deletions

diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index f63b58f515..ad79e279be 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -2173,7 +2173,7 @@
     \value SplashScreen Indicates that the window is a splash screen.
                         This is the default type for QSplashScreen.
 
-    \value Desktop  Indicates that this widget is the desktop. This
+    \omitvalue Desktop  Indicates that this widget is the desktop. This
                     is the type for QDesktopWidget.
 
     \value SubWindow  Indicates that this widget is a sub-window, such
diff --git a/src/gui/accessible/qaccessible.cpp b/src/gui/accessible/qaccessible.cpp
index 22d94fad79..7fca4fb0f7 100644
--- a/src/gui/accessible/qaccessible.cpp
+++ b/src/gui/accessible/qaccessible.cpp
@@ -707,7 +707,7 @@ QAccessibleInterface *QAccessible::queryAccessibleInterface(QObject *object)
         QAccessiblePlugin *factory = qAccessiblePlugins()->value(cn);
         if (factory) {
             QAccessibleInterface *result = factory->create(cn, object);
-            if (result) {   // Need this condition because of QDesktopScreenWidget
+            if (result) {
                 QAccessibleCache::instance()->insert(object, result);
                 Q_ASSERT(QAccessibleCache::instance()->containsObject(object));
             }
diff --git a/src/widgets/accessible/qaccessiblewidgetfactory.cpp b/src/widgets/accessible/qaccessiblewidgetfactory.cpp
index d59da86076..4c6a6fdecb 100644
--- a/src/widgets/accessible/qaccessiblewidgetfactory.cpp
+++ b/src/widgets/accessible/qaccessiblewidgetfactory.cpp
@@ -222,8 +222,6 @@ QAccessibleInterface *qAccessibleFactory(const QString &classname, QObject *obje
         iface = new QAccessibleDockWidget(widget);
 #endif
 
-    } else if (classname == QLatin1String("QDesktopScreenWidget")) {
-        iface = nullptr;
     } else if (classname == QLatin1String("QWidget")) {
         iface = new QAccessibleWidget(widget);
     } else if (classname == QLatin1String("QWindowContainer")) {
diff --git a/src/widgets/kernel/qdesktopwidget.cpp b/src/widgets/kernel/qdesktopwidget.cpp
index 797980c7c5..db8c33c498 100644
--- a/src/widgets/kernel/qdesktopwidget.cpp
+++ b/src/widgets/kernel/qdesktopwidget.cpp
@@ -48,26 +48,6 @@
 
 QT_BEGIN_NAMESPACE
 
-QDesktopScreenWidget::QDesktopScreenWidget(QScreen *screen, const QRect &geometry)
-    : QWidget(nullptr, Qt::Desktop)
-{
-    setVisible(false);
-    if (QWindow *winHandle = windowHandle())
-        winHandle->setScreen(screen);
-    setGeometry(geometry);
-}
-
-QScreen *QDesktopScreenWidget::screen() const
-{
-    const QDesktopWidgetPrivate *desktopWidgetP
-        = static_cast<const QDesktopWidgetPrivate *>(qt_widget_private(QApplication::desktop()));
-    for (auto it : qAsConst(desktopWidgetP->screenWidgets)) {
-        if (it.second == this)
-            return it.first;
-    }
-    return nullptr;
-}
-
 QDesktopWidgetPrivate::~QDesktopWidgetPrivate()
 {
     qDeleteAll(screenWidgets.values());
@@ -81,15 +61,19 @@ void QDesktopWidgetPrivate::updateScreens()
     // Re-build our screens list. This is the easiest way to later compute which signals to emit.
     // Create new screen widgets as necessary.
     // Furthermore, we note which screens have changed, and compute the overall virtual geometry.
-    QFlatMap<QScreen*, QDesktopScreenWidget*> newScreenWidgets;
+    QFlatMap<QScreen*, QWidget*> newScreenWidgets;
     QRegion virtualGeometry;
 
     for (QScreen *screen : screenList) {
         const QRect screenGeometry = screen->geometry();
-        QDesktopScreenWidget *screenWidget = screenWidgets.value(screen);
+        QWidget *screenWidget = screenWidgets.value(screen);
         if (!screenWidget) {
             // a new screen, create a widget and connect the signals.
-            screenWidget = new QDesktopScreenWidget(screen, screenGeometry);
+            screenWidget = new QWidget(nullptr, Qt::Desktop);
+            screenWidget->setVisible(false);
+            screenWidget->setScreen(screen);
+            screenWidget->setGeometry(screenGeometry);
+            screenWidget->setObjectName(QLatin1String("qt_desktop_widget_%1").arg(screen->name()));
             QObjectPrivate::connect(screen, &QScreen::geometryChanged,
                                     this, &QDesktopWidgetPrivate::updateScreens, Qt::QueuedConnection);
             QObjectPrivate::connect(screen, &QObject::destroyed,
@@ -105,7 +89,7 @@ void QDesktopWidgetPrivate::updateScreens()
     Q_ASSERT(screenWidgets.size() == screenList.length());
     q->setGeometry(virtualGeometry.boundingRect());
 
-    // Delete the QDesktopScreenWidget that are not used any more.
+    // Delete the screen widgets that are not used any more.
     for (auto it : qAsConst(newScreenWidgets)) {
         if (!screenWidgets.contains(it.first))
             delete it.second;
diff --git a/src/widgets/kernel/qdesktopwidget_p.h b/src/widgets/kernel/qdesktopwidget_p.h
index f0664794bc..c17036b9d0 100644
--- a/src/widgets/kernel/qdesktopwidget_p.h
+++ b/src/widgets/kernel/qdesktopwidget_p.h
@@ -77,26 +77,18 @@ private:
     friend class QApplicationPrivate;
 };
 
-class QDesktopScreenWidget : public QWidget {
-    Q_OBJECT
-public:
-    explicit QDesktopScreenWidget(QScreen *, const QRect &geometry);
-
-    QScreen *screen() const;
-};
-
 class QDesktopWidgetPrivate : public QWidgetPrivate {
     Q_DECLARE_PUBLIC(QDesktopWidget)
 
 public:
     ~QDesktopWidgetPrivate();
     void updateScreens();
-    QDesktopScreenWidget *widgetForScreen(QScreen *qScreen) const
+    QWidget *widgetForScreen(QScreen *qScreen) const
     {
         return screenWidgets.value(qScreen);
     }
 
-    QFlatMap<QScreen*, QDesktopScreenWidget*> screenWidgets;
+    QFlatMap<QScreen*, QWidget*> screenWidgets;
 };
 
 QT_END_NAMESPACE
diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 2bfb4c3096..38f51e270e 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -42,7 +42,6 @@
 #include "qapplication_p.h"
 #include "qbrush.h"
 #include "qcursor.h"
-#include "qdesktopwidget_p.h"
 #include "qevent.h"
 #include "qlayout.h"
 #if QT_CONFIG(menu)
@@ -990,13 +989,6 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
     if (allWidgets)
         allWidgets->insert(q);
 
-    QScreen *targetScreen = nullptr;
-    if (parentWidget && parentWidget->windowType() == Qt::Desktop) {
-        const QDesktopScreenWidget *sw = qobject_cast<const QDesktopScreenWidget *>(parentWidget);
-        targetScreen = sw ? sw->screen() : nullptr;
-        parentWidget = nullptr;
-    }
-
     q->data = &data;
 
 #if QT_CONFIG(thread)
@@ -1006,12 +998,6 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
     }
 #endif
 
-    if (targetScreen) {
-        topData()->initialScreen = targetScreen;
-        if (QWindow *window = q->windowHandle())
-            window->setScreen(targetScreen);
-    }
-
     data.fstrut_dirty = true;
 
     data.winid = 0;
@@ -2424,8 +2410,7 @@ bool QWidgetPrivate::setScreen(QScreen *screen)
         return false;
     const QScreen *currentScreen = windowHandle() ? windowHandle()->screen() : nullptr;
     if (currentScreen != screen) {
-        if (!windowHandle()) // Try to create a window handle if not created.
-            createWinId();
+        topData()->initialScreen = screen;
         if (windowHandle())
             windowHandle()->setScreen(screen);
         return true;
@@ -2514,6 +2499,24 @@ QScreen *QWidget::screen() const
     return QGuiApplication::primaryScreen();
 }
 
+/*!
+    Sets the screen on which the widget should be shown to \a screen.
+
+    Setting the screen only makes sense for windows. If necessary, the widget's
+    window will get recreated on \a screen.
+
+    \note If the screen is part of a virtual desktop of multiple screens,
+    the window will not move automatically to \a newScreen. To place the
+    window relative to the screen, use the screen's topLeft() position.
+
+    \sa QWindow::setScreen
+*/
+void QWidget::setScreen(QScreen *screen)
+{
+    Q_D(QWidget);
+    d->setScreen(screen);
+}
+
 #ifndef QT_NO_STYLE_STYLESHEET
 
 /*!
@@ -10508,8 +10511,7 @@ 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
-        const QDesktopScreenWidget *sw = qobject_cast<const QDesktopScreenWidget *>(newparent);
-        targetScreen = sw ? sw->screen() : nullptr;
+        targetScreen = newparent->screen();
         newparent = nullptr;
     }
 
@@ -10539,10 +10541,8 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
 
     if (!newparent) {
         f |= Qt::Window;
-        if (!targetScreen) {
-            if (parent)
-                targetScreen = q->parentWidget()->window()->screen();
-        }
+        if (parent)
+            targetScreen = q->parentWidget()->window()->screen();
     }
 
     bool explicitlyHidden = q->testAttribute(Qt::WA_WState_Hidden) && q->testAttribute(Qt::WA_WState_ExplicitShowHide);
diff --git a/src/widgets/kernel/qwidget.h b/src/widgets/kernel/qwidget.h
index 2bfff96252..6f4c709c10 100644
--- a/src/widgets/kernel/qwidget.h
+++ b/src/widgets/kernel/qwidget.h
@@ -598,11 +598,10 @@ public:
 
     QWindow *windowHandle() const;
     QScreen *screen() const;
+    void setScreen(QScreen *);
 
     static QWidget *createWindowContainer(QWindow *window, QWidget *parent=nullptr, Qt::WindowFlags flags=Qt::WindowFlags());
 
-    friend class QDesktopScreenWidget;
-
 Q_SIGNALS:
     void windowTitleChanged(const QString &title);
     void windowIconChanged(const QIcon &icon);
diff --git a/src/widgets/widgets/qmenu.cpp b/src/widgets/widgets/qmenu.cpp
index 74420ecb2c..9528be17b7 100644
--- a/src/widgets/widgets/qmenu.cpp
+++ b/src/widgets/widgets/qmenu.cpp
@@ -2377,7 +2377,7 @@ void QMenuPrivate::popup(const QPoint &p, QAction *atAction, PositionFunction po
     updateLayoutDirection();
 
     // Ensure that we get correct sizeHints by placing this window on the correct screen.
-    // However if the QMenu was constructed with a QDesktopScreenWidget as its parent,
+    // However if the QMenu was constructed with a Qt::Desktop widget as its parent,
     // then initialScreenIndex was set, so we should respect that for the lifetime of this menu.
     // However if eventLoop exists, then exec() already did this by calling createWinId(); so leave it alone. (QTBUG-76162)
     if (!eventLoop) {
diff --git a/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp b/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
index 6e304f2152..ae91685697 100644
--- a/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
+++ b/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
@@ -196,6 +196,7 @@ private slots:
     void activation();
 #endif
     void reparent();
+    void setScreen();
     void windowState();
     void showMaximized();
     void showFullScreen();
@@ -2943,6 +2944,29 @@ void tst_QWidget::reparent()
     QTRY_COMPARE(childTLW.pos(), tlwPos);
 }
 
+void tst_QWidget::setScreen()
+{
+    const auto screens = QApplication::screens();
+    if (screens.count() < 2)
+        QSKIP("This test tests nothing on a machine with a single screen.");
+
+    QScreen *screen0 = screens.at(0);
+    QScreen *screen1 = screens.at(1);
+
+    QWidget window;
+    window.setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint);
+    window.setScreen(screen0);
+    QCOMPARE(window.screen(), screen0);
+    window.setScreen(screen1);
+    QCOMPARE(window.screen(), screen1);
+
+    // calling setScreen on a widget that is not a window does nothing
+    QWidget child(&window);
+    const QScreen *childScreen = child.screen();
+    child.setScreen(childScreen == screen0 ? screen1 : screen0);
+    QCOMPARE(child.screen(), childScreen);
+}
+
 // Qt/Embedded does it differently.
 void tst_QWidget::icon()
 {
diff --git a/tests/manual/widgets/kernel/CMakeLists.txt b/tests/manual/widgets/kernel/CMakeLists.txt
index 9be003391d..b4443fa3e2 100644
--- a/tests/manual/widgets/kernel/CMakeLists.txt
+++ b/tests/manual/widgets/kernel/CMakeLists.txt
@@ -3,3 +3,4 @@
 # add_subdirectory(qtooltip) # special case broken in dev
 add_subdirectory(sizeonhide)
 add_subdirectory(layoutreplace)
+add_subdirectory(setscreen)
diff --git a/tests/manual/widgets/kernel/kernel.pro b/tests/manual/widgets/kernel/kernel.pro
index 5dbd25f8bd..236c9ff9d6 100644
--- a/tests/manual/widgets/kernel/kernel.pro
+++ b/tests/manual/widgets/kernel/kernel.pro
@@ -1,2 +1,2 @@
 TEMPLATE = subdirs
-SUBDIRS = qtooltip sizeonhide layoutreplace
+SUBDIRS = qtooltip sizeonhide layoutreplace setscreen
diff --git a/tests/manual/widgets/kernel/setscreen/CMakeLists.txt b/tests/manual/widgets/kernel/setscreen/CMakeLists.txt
new file mode 100644
index 0000000000..cf31341d7c
--- /dev/null
+++ b/tests/manual/widgets/kernel/setscreen/CMakeLists.txt
@@ -0,0 +1,18 @@
+# Generated from setscreen.pro.
+
+#####################################################################
+## setscreen Binary:
+#####################################################################
+
+qt_add_manual_test(setscreen
+    GUI
+    SOURCES
+        main.cpp
+    PUBLIC_LIBRARIES
+        Qt::CorePrivate
+        Qt::Gui
+        Qt::Widgets
+)
+
+#### Keys ignored in scope 1:.:.:setscreen.pro:<TRUE>:
+# TEMPLATE = "app"
diff --git a/tests/manual/widgets/kernel/setscreen/main.cpp b/tests/manual/widgets/kernel/setscreen/main.cpp
new file mode 100644
index 0000000000..70ec067d3d
--- /dev/null
+++ b/tests/manual/widgets/kernel/setscreen/main.cpp
@@ -0,0 +1,156 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QtWidgets>
+
+class ScreenWidget : public QWidget
+{
+public:
+    ScreenWidget(QWidget *parent)
+    : QWidget(parent, Qt::Window)
+    {
+        textEdit = new QTextEdit;
+        textEdit->setReadOnly(true);
+
+        QHBoxLayout *layout = new QHBoxLayout;
+        layout->addWidget(textEdit);
+        setLayout(layout);
+    }
+
+    void updateText()
+    {
+        QString text = "<html><body>";
+        text += QString("<p>Screen: %1\n</p>").arg(screen()->name());
+        text += QString("<p>DPR: %1\n</p>").arg(screen()->devicePixelRatio());
+        text += QString("</body></html>");
+
+        textEdit->setText(text);
+    }
+
+private:
+    QTextEdit *textEdit;
+};
+
+class Controller : public QDialog
+{
+    Q_OBJECT
+
+public:
+    Controller()
+    {
+        QPushButton *screenButton = new QPushButton;
+        screenButton->setText("Show on Screen");
+        screenButton->setEnabled(false);
+        connect(screenButton, &QAbstractButton::clicked, this, &Controller::setScreen);
+
+        QPushButton *desktopButton = new QPushButton;
+        desktopButton->setText("Show on Desktop");
+        desktopButton->setEnabled(false);
+        connect(desktopButton, &QAbstractButton::clicked, this, &Controller::setDesktop);
+
+        QPushButton *exitButton = new QPushButton;
+        exitButton->setText("E&xit");
+        connect(exitButton, &QAbstractButton::clicked, QApplication::instance(), &QCoreApplication::quit);
+
+        QHBoxLayout *actionLayout = new QHBoxLayout;
+        actionLayout->addWidget(screenButton);
+        actionLayout->addWidget(desktopButton);
+        actionLayout->addWidget(exitButton);
+
+        QGroupBox *radioGroup = new QGroupBox;
+        radioGroup->setTitle(QLatin1String("Select target screen"));
+
+        QVBoxLayout *groupLayout = new QVBoxLayout;
+        const auto screens = QGuiApplication::screens();
+        int count = 0;
+        for (const auto &screen : screens) {
+            QRadioButton *choice = new QRadioButton;
+            choice->setText(QString(QLatin1String("%1: %2")).arg(count).arg(screen->name()));
+            connect(choice, &QAbstractButton::toggled, this, [=](bool on){
+                if (on)
+                    targetScreen = count;
+                screenButton->setEnabled(targetScreen != -1);
+                desktopButton->setEnabled(targetScreen != -1);
+            });
+            groupLayout->addWidget(choice);
+            ++count;
+        }
+        radioGroup->setLayout(groupLayout);
+
+        QVBoxLayout *layout = new QVBoxLayout;
+        layout->addWidget(radioGroup);
+        layout->addLayout(actionLayout);
+        setLayout(layout);
+    }
+
+private slots:
+    void setScreen()
+    {
+        QScreen *screen = QGuiApplication::screens().at(targetScreen);
+        if (!widget) {
+            widget = new ScreenWidget(this);
+            widget->setAttribute(Qt::WA_DeleteOnClose);
+            widget->setWindowTitle("Normal Window");
+        }
+        widget->setScreen(screen);
+        widget->show();
+        widget->updateText();
+    }
+
+    void setDesktop()
+    {
+        QScreen *screen = QGuiApplication::screens().at(targetScreen);
+        QWidget *desktop = QApplication::desktop(screen);
+        if (!desktopChild) {
+            desktopChild = new ScreenWidget(desktop);
+            desktopChild->setAttribute(Qt::WA_DeleteOnClose);
+            desktopChild->setWindowTitle("Child of a Desktop");
+        } else {
+            desktopChild->setParent(desktop);
+        }
+        desktopChild->show();
+        desktopChild->updateText();
+    }
+
+private:
+    QPointer<ScreenWidget> widget = nullptr;
+    QPointer<ScreenWidget> desktopChild = nullptr;
+    int targetScreen = -1;
+};
+
+int main(int argc, char *argv[])
+{
+    QApplication app(argc, argv);
+
+    Controller controller;
+    controller.show();
+
+    return app.exec();
+}
+
+#include "main.moc"
diff --git a/tests/manual/widgets/kernel/setscreen/setscreen.pro b/tests/manual/widgets/kernel/setscreen/setscreen.pro
new file mode 100644
index 0000000000..f06006ea3a
--- /dev/null
+++ b/tests/manual/widgets/kernel/setscreen/setscreen.pro
@@ -0,0 +1,3 @@
+TEMPLATE = app
+SOURCES = main.cpp
+QT += widgets