This would mimic the effect you can see here in the iOS scrollbar.
I think this may require changes to the gio
layout.List type's behavior so that it can drag content beyond the edges of the screen. Right now it's impossible to drag the scrollable list content beyond its own boundaries.
Of course, the scroll bar isn't fundamentally tied to the use-case of
layout.List (or our API isn't), so perhaps we should build a different scrollable content area type that can demo this capability? I think it would be useful to have a fixed-dimension window onto a larger space in Gio.