Fix the focus frame on Mac

The Mac style uses focus in/out events (sent by QApplication) in order
to update the focus frame. If a proxy style is installed, these events
never reach the Mac style object.

Amends 5d8a7652b995124495ef4f4a43fd8cf461367d62 , by making the
forwarding limited to the events for which we want it to happen, and not
just *any* event.

Fixes: QTBUG-109375
Change-Id: I6df49aa81d6ebb8dbaf00b9ba99e2a7c006e1181
Pick-to: 6.4 6.5
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/styles/qproxystyle.cpp

@@ -348,7 +348,18 @@ void QProxyStyle::unpolish(QApplication *app)
  */
 bool QProxyStyle::event(QEvent *e)
 {
-    // ### Qt 7: remove this override
+    Q_D (QProxyStyle);
+
+    switch (e->type()) {
+    // The Mac style relies on these events in order to set the focus frame
+    case QEvent::FocusIn:
+    case QEvent::FocusOut:
+        d->ensureBaseStyle();
+        return QApplication::sendEvent(d->baseStyle, e);
+    default:
+        break;
+    }
+
     return QCommonStyle::event(e);
 }