Widgets: add QWidgetPrivate::deepestFocusProxy()

Factor out searching for the deepest focus proxy
in to a private function. Other than being called
from QWidget::setFocus(), this function will also be
used in subsequent patches.

Change-Id: I4450a42e362eccb64f8a88c7ea83b415101973b9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -6523,9 +6523,9 @@ void QWidget::setFocus(Qt::FocusReason reason)
     if (!isEnabled())
         return;
 
-    QWidget *f = this;
-    while (f->d_func()->extra && f->d_func()->extra->focus_proxy)
-        f = f->d_func()->extra->focus_proxy;
+    QWidget *f = d_func()->deepestFocusProxy();
+    if (!f)
+        f = this;
 
     if (QApplication::focusWidget() == f
 #if 0 // Used to be included in Qt4 for Q_WS_WIN
@@ -6622,6 +6622,27 @@ void QWidget::setFocus(Qt::FocusReason reason)
     }
 }
 
+
+/*!\internal
+ * A focus proxy can have its own focus proxy, which can have its own
+ * proxy, and so on. This helper function returns the widget that sits
+ * at the bottom of the proxy chain, and therefore the one that should
+ * normally get focus if this widget receives a focus request.
+ */
+QWidget *QWidgetPrivate::deepestFocusProxy() const
+{
+    Q_Q(const QWidget);
+
+    QWidget *focusProxy = q->focusProxy();
+    if (!focusProxy)
+        return nullptr;
+
+    while (QWidget *nextFocusProxy = focusProxy->focusProxy())
+        focusProxy = nextFocusProxy;
+
+    return focusProxy;
+}
+
 void QWidgetPrivate::setFocus_sys()
 {
     Q_Q(QWidget);

src/widgets/kernel/qwidget_p.h

@@ -369,6 +369,7 @@ public:
     void lower_sys();
     void stackUnder_sys(QWidget *);
 
+    QWidget *deepestFocusProxy() const;
     void setFocus_sys();
     void updateFocusChild();