When you close Bookmarks sidebar (or change to other sidebar panel), it memorizes sidebar's vertical scroll position. And restores it at sidebar re-open.
Additionally, History sidebar is supported too. (but this support is only sub feature.)
This add-on works each window, each session individually.
(It means it does not keep same scroll position in every window every session.)
This add-on aims to simply keep scrolled item index.
It is not assumed to add, delete, insert, sort or so in other window.
If you do so, restored position may be incorrect.
History sidebar support is only sub feature.
Changing sort type after search may cause incorrect position.