The button to collapse the sidebar should always be visible and easy clickable.
On my resolution a scrollbar appears if two users are shown.
This scrollbar is overlapping with the button making it very hard to see and use.
The scrollbar of the sidebar might be optimized a little bit.
One possible solution would be to hide the scrollbar if its not in use.
Something like this, could be implemented:
https://nicescroll.areaaperta.com/demo/
That the scroll bar is hidden when it is inactive is in my opinion very good for the usability of Sidebar.
Just go to an account of another Steemit member. This is now displayed under your own account and the scrollbar appears.