Feature Request Easier to extend elements

UI Toolkit is really awesome so far but I do hit a lot of blockers because of members marked internal or private when i am trying to extend on it an existing element. This is mostly because I am working on runtime UI rather than editor side UI.

I'd like if you would go through the controls made so far and look at where they could be opened up more. I'll use the Slider as example but there are other.

I am simply trying to add a fill effect to its track. So from the left up to where the dragger element is would be a fill and to the right the normal track background colour.

I thought I'd derive a new class from Slider and then add my fill element in the constructor to the track element. That worked fine but next I need to figure out how to actually update the fill. It would have been so much easier if I could simply override UpdateDragElementPosition() and then just call base.UpdateDragElementPosition() before doing the fill's update calculations.

I dunno, maybe i am just asking for things that can easily be done and I'm just not seeing the solution or there are good reasons for all these members marked as internal or private or not virtual.

Back to figuring out how to do the update on the fill before I have to copy all the source of BaseSlider and Slider to spin my own version :s

[edit] I ended up registering a Value Changed callback, which works fine. So I guess what I am asking for is pointless when one look at a solution like that.