Qt menu bar appears inappropriately when hiding and re-showing it.

I have a menu bar in my Qt application, in which there is a QMainWindow with a menu bar.

I call
ui->menuBar->hide();
and later on
ui->menuBar->show();

After this, the menu bar appears inappropriately, like the menu bars of desktop applications at the top of the application window. This is undersirable on Maemo 5.