QWidgetWindow::setFocusToTarget: Respect focus policies and proxies

When calling QWidgetWindowPrivate::setFocusToTarget with Prev or Next
target parameter, we were just setting focus to the
next/prevInFocusChain() of the window's focusWidget(). This will bypass
focus proxies and focus policies of the widget, which is wrong as it
can end up giving eg: tab focus to a widget that does not have such
focus policy.
To fix, we should instead call QWidget::focusNextPrevChild which
determines the right next/prev in the TAB focus chain. As this is a
protected member of QWidget, implement a "wrapper" for it in
QWidgetWindow which is a friend class of QWidget.

Pick-to: 6.8
Task-number: QTBUG-121789
Change-Id: I1f4f5d85e7552926580906fdef6f0a456fe7486c
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: MohammadHossein Qanbari <mohammad.qanbari@qt.io>

src/widgets/kernel/qwidgetwindow.cpp

@@ -82,31 +82,26 @@ class QWidgetWindowPrivate : public QWindowPrivate
         QWidget *widget = q->widget();
         if (!widget)
             return;
-        QWidget *newFocusWidget = nullptr;
 
         switch (target) {
-        case FocusTarget::First:
-            newFocusWidget = q->getFocusWidget(QWidgetWindow::FirstFocusWidget);
-            break;
-        case FocusTarget::Last:
-            newFocusWidget = q->getFocusWidget(QWidgetWindow::LastFocusWidget);
-            break;
+        case FocusTarget::Prev:
         case FocusTarget::Next: {
             QWidget *focusWidget = widget->focusWidget() ? widget->focusWidget() : widget;
-            newFocusWidget = focusWidget->nextInFocusChain() ? focusWidget->nextInFocusChain() : focusWidget;
-            break;
+            q->focusNextPrevChild(focusWidget, target == FocusTarget::Next);
+            return;
         }
-        case FocusTarget::Prev: {
-            QWidget *focusWidget = widget->focusWidget() ? widget->focusWidget() : widget;
-            newFocusWidget = focusWidget->previousInFocusChain() ? focusWidget->previousInFocusChain() : focusWidget;
+        case FocusTarget::First:
+        case FocusTarget::Last: {
+            QWidgetWindow::FocusWidgets fw = target == FocusTarget::First
+                ? QWidgetWindow::FirstFocusWidget
+                : QWidgetWindow::LastFocusWidget;
+            if (QWidget *newFocusWidget = q->getFocusWidget(fw))
+                newFocusWidget->setFocus(reason);
             break;
         }
         default:
             break;
         }
-
-        if (newFocusWidget)
-            newFocusWidget->setFocus(reason);
     }
 
     QRectF closestAcceptableGeometry(const QRectF &rect) const override;
@@ -233,6 +228,12 @@ void QWidgetWindow::setNativeWindowVisibility(bool visible)
     d->QWindowPrivate::setVisible(visible);
 }
 
+void QWidgetWindow::focusNextPrevChild(QWidget *widget, bool next)
+{
+    Q_ASSERT(widget);
+    widget->focusNextPrevChild(next);
+}
+
 static inline bool shouldBePropagatedToWidget(QEvent *event)
 {
     switch (event->type()) {

src/widgets/kernel/qwidgetwindow_p.h

@@ -46,6 +46,8 @@ class QWidgetWindow : public QWindow
 
     QObject *focusObject() const override;
     void setNativeWindowVisibility(bool visible);
+    static void focusNextPrevChild(QWidget *widget, bool next);
+
 protected:
     bool event(QEvent *) override;