[REG: Qt 5.4.1-> Qt 5.4.2] Floating QDockWidget with custom titlebar-widget flickers while resizing (Mac OS X)

We've compiled "Main Window" from the Qt examples. The blue QDockWidget is using a customized tool-bar (subclass of QWidget). When the dock-widget is undocked it flickers while resizing it.
The other QDockWidgets within the example work like a charm. If we outcomment

BlueTitleBar *titlebar = new BlueTitleBar(swatch);
swatch->setTitleBarWidget(titlebar);
connect(swatch, SIGNAL(topLevelChanged(bool)), titlebar, SLOT(updateMask()));
connect(swatch, SIGNAL(featuresChanged(QDockWidget::DockWidgetFeatures)), titlebar, SLOT(updateMask()), Qt::QueuedConnection);

also the blue widget's resize-behaviour is fine.