Call Scrollbar::setValue and implement the two update methods.
One remaining question is if I need to add some special handling to hide the scrollbar if they are not needed?
Created attachment 15714 [details]
Fully implement PlatformScrollbar
Call setValue from a closure connected to value-changed of GtkAdjustment. Set GtkAdjustment::value from updateThumbPosition and use updateThumbProportion to set min,max,page_size,step- and page-increment.
This was landed in r24926.