This is a very simple problem to reproduce. Create a new project with Qt Creator with a
Yes, it's possible. All you need is setting margins of that
0. (note that you should first set up a layout)
In properties panel set the following properties to
Also note that using a
QMainWindow) as your main app window is perfectly valid, so if your app has only one widget, you won't need a
QMainWindow at all. It's enough to
show that widget.