There’s already a button to close/open the SideBar.
No there isn't. Check my screencap.
The button only appears if I back up all the way to the root. That may involve up to seven taps in my case. Then, when I open the sidebar again, I have to navigate all the way back in order to tap my next bookmark.
If there was a button (and even if it did reset my position), then it would at least spare me the hassle to go back all the way to the root just to close the damn thing.