as I understand it, the need to have a dynamic slider doesn’t exist on mobile devices, since the sidebar is now on top, with width dictated by the device, and height dictated by the content, so it’s simply an open/close toggle?
In which case, I think a horizontal bar full-width, to seperate content from the sidebar top content, with a tappable button in the middle, that collapses the top content and has just the bar along the top, with central button to expand again.
…I’m not sure this is great as it moves the control from the top of the top-content, to the bottom of it. Otoh, when the content is hidden, the current ‘show’ icon basically overlaps the topmost close ‘x’, so moving the tappable ‘show’ away from that would be a win IMHO.