widgets: Handle all flush tracking via markNeedsFlush

Change-Id: I3652c09012e36468ef90870637988b3fe8c5e735
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/widgets/kernel/qwidgetrepaintmanager.cpp

@@ -964,8 +964,11 @@ void QWidgetRepaintManager::paintAndFlush()
     }
 #endif
 
-    // Always flush repainted areas
-    topLevelNeedsFlush += toClean;
+    // Always flush repainted areas. FIXME: We should mark individual widgets,
+    // not the top level widget unconditionally, as this results in always
+    // flushing the top level widget, even if the painted region is entirely
+    // within a native child.
+    markNeedsFlush(tlw, toClean, QPoint());
 
     store->beginPaint(toClean);