I can think of one way to do it, but your widget will have to be fixed size.

This is based on the property of widgets to clip their children.
You must have a container widget that has initially an empty size and a the widget you want to display(at real size). Add the widget to the empty widget in a vertical layout.

Next all you have to do is to increase the size of the container and move it accordingly(something to look like a slide) until the container
reaches the size of the widget.
So, this should be enough.

Regards