On macOS, the button to expand/retract the right sidebar conveniently doesn’t move when the sidebar opens, making it easy to quickly open and close it without moving the mouse. The left sidebar button, on the other hand, currently moves with the sidebar’s opening and closing.
Proposed solution
Make the left sidebar button remain stationary, with sidebar-related buttons appearing to its right (just as the right sidebar’s buttons appear to its left). That would enable quick peeks at left sidebar content without having to break concentration and move the mouse to chase the button. It would also make sidebar-related UI behavior more consistent and predictable in general.
Yes, at least on macOS that would keep the button next to the OS window management buttons. Basically, it would remain where it is when the sidebar is in its collapsed state.