How to create a scrollbar

I was working on a simple application and I would really need to have a scrollbar to move around inside big file and display some file-based content on and uiArea.

The uiScollingArea is not an option because it required to draw the whole image and it will be too inefficient, I really need a scrollbar.

I understood that currently there are no scrollbars in libui but they exists in GTK+ with some simple APIs. As for windows I don't know exactly but I guess they should exists.

As a last resort I may implement scrollbars in libui by myself but I would like to have some advice.
Any specific reason they are not already in libui ?
Some idea about how to implement them in Windows ?

If other people are interested too I would like to collaborate to make a good implementation.