A qt widget with fully transparent background

By default in Qt4, a QWidget will draw nothing for its own background, and only its children will be drawn. If you want to override that, you specifically have to tell the widget to draw its background via one of its properties. Note that some widgets derived from QWidget will automatically draw backgrounds.

You should be able to do all the drawing customisation you need by changing the style of your widget i think

MyWidget {background-color: none;}

should work, stylesheets can very easily be tested in the designer