This looks strange. The problem is created by QMenuBar. As QMainWindow's default QMenuBar is hidden and when you create a new maximized the MDI child with menubar hidden, it is causing this button's missing, (in my case the Qt Icon on the left of menus is also missing, on Windows Vista).

If you set default menu bar visible before creating the MDI child it works ok. Just by calling menuBar()->show() in ctor, or mainWin.menuBar()->show() before mainWin.newFile() in main.cpp

I guess QMainWindow's layout manager is not handing menu bar hidden scenario properly, which is causing the widgets to be overlapping (or something of that sort which is causing geometry problems).