Qt::Tool
Indicates that the widget is a tool window. A tool window is often a small window with a smaller than usual title bar and decoration, typically used for collections of tool buttons. If there is a parent, the tool window will always be kept on top of it. If there isn't a parent, you may consider using Qt::WindowStaysOnTopHint as well. If the window system supports it, a tool window can be decorated with a somewhat lighter frame. It can also be combined with Qt::FramelessWindowHint.

QWidget *w(new QWidget());
w->setWindowFlags(Qt::Tool | Qt::FramelessWindowHint);
w->show();

Can be checked with widgets->windowflags example