Add screen() accessor to QWidget

Base it on QWidgetPrivate::associatedScreen(), but make a larger
effort to find a screen in case the widget is not shown yet.

Rename QDesktopScreenWidget::screen() to something else to avoid
clashes.

Task-number: QTBUG-62094
Task-number: QTBUG-53022
Change-Id: I36ba5ef5f0645a4ac89da0b38a391f7057b2f49c
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/kernel/qdesktopwidget.cpp

@@ -111,7 +111,7 @@ const QRect QDesktopWidgetPrivate::availableGeometry(const QWidget *widget)
 QDesktopScreenWidget *QDesktopWidgetPrivate::widgetForScreen(QScreen *qScreen) const
 {
     foreach (QDesktopScreenWidget *widget, screens) {
-        if (widget->screen() == qScreen)
+        if (widget->assignedScreen() == qScreen)
             return widget;
     }
     return nullptr;

src/widgets/kernel/qdesktopwidget_p.h

@@ -68,7 +68,7 @@ public:
     int screenNumber() const;
     void setScreenGeometry(const QRect &geometry);
 
-    QScreen *screen() const { return m_screen.data(); }
+    QScreen *assignedScreen() const { return m_screen.data(); }
     QRect screenGeometry() const { return m_geometry; }
 
 private:

src/widgets/kernel/qwidget.cpp

@@ -2445,7 +2445,7 @@ WId QWidget::effectiveWinId() const
 
     \since 5.0
 
-    \sa winId()
+    \sa winId(), screen()
 */
 QWindow *QWidget::windowHandle() const
 {
@@ -2453,6 +2453,29 @@ QWindow *QWidget::windowHandle() const
     return d->windowHandle();
 }
 
+/*!
+    Returns the screen the widget is on.
+
+    \since 5.14
+
+    \sa windowHandle()
+*/
+QScreen *QWidget::screen() const
+{
+    Q_D(const QWidget);
+    if (auto associatedScreen = d->associatedScreen())
+        return associatedScreen;
+    if (auto topLevel = window()) {
+        if (auto topData = qt_widget_private(topLevel)->topData()) {
+            if (auto initialScreen = QGuiApplicationPrivate::screen_list.value(topData->initialScreenIndex))
+                return initialScreen;
+        }
+        if (auto screenByPos = QGuiApplication::screenAt(topLevel->geometry().center()))
+            return screenByPos;
+    }
+    return QGuiApplication::primaryScreen();
+}
+
 #ifndef QT_NO_STYLE_STYLESHEET
 
 /*!

src/widgets/kernel/qwidget.h

@@ -85,6 +85,7 @@ class QDragEnterEvent;
 class QDragMoveEvent;
 class QDragLeaveEvent;
 class QDropEvent;
+class QScreen;
 class QShowEvent;
 class QHideEvent;
 class QIcon;
@@ -601,6 +602,7 @@ public:
     QBackingStore *backingStore() const;
 
     QWindow *windowHandle() const;
+    QScreen *screen() const;
 
     static QWidget *createWindowContainer(QWindow *window, QWidget *parent=nullptr, Qt::WindowFlags flags=Qt::WindowFlags());
 

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -1139,6 +1139,7 @@ void tst_QWidget::visible()
     QVERIFY( !childWidget->isVisible() );
 
     testWidget->show();
+    QVERIFY(testWidget->screen());
     QVERIFY( testWidget->isVisible() );
     QVERIFY( childWidget->isVisible() );