The menu bar is different between OS?

My desktop Application has the menu bar. It is OK at Ubuntu, but when at Fedora(28) and Window (7), the menu bar is stay the top of window. So, The interface is push down and unable to show the bottom of it.
How to make Menubar at Fedora & Window is the same Ubuntu style or opposite?

It sounds like you have not set up your MainWindow with a correct layout for the central window, or you have set a fixed size for the MainWindow which prevents it from adapting to the different OS. The effect you are seeing should not happen if you have used the layout correctly and have not set a fixed size.