When in call the winId(), show full screen will has a double toolbar,
if remove winId(), show full screen work fine.
i have attached the screen and the demo project to reproduce this issue.

MainWindow::MainWindow()
: textEdit(new QTextEdit)

{ setCentralWidget(textEdit); createActions(); createStatusBar(); createDockWindows(); setWindowTitle(tr("Dock Widgets")); newLetter(); setUnifiedTitleAndToolBarOnMac(true); textEdit->winId(); }