QDialog is not top-level

A QDialog should be top-level. This is also stated in the Qt5 documentation: "A dialog window is a top-level window mostly used for short-term tasks and brief communications with the user." (http://doc.qt.io/qt-5/qdialog.html) But since Qt5 this is not the case anymore. Run attached file with Qt4 and it behaves as stated, but running it with Qt5 the dialog disappears in the background as soon as the main window has the focus.