Scrollbars in bookmarks library play hide 'N seek

This may be an old problem. In Linux, in ''bookmarks library'', the scrollbar thumbs behave differently than in browser ''web pages''. Even in new profile. This isn't a new issue for v157. Same behavior in any theme (any ''Fx themes'', and / or diff '''Nix system themes''). I set scrollbars to disappear when not used or being hovered. '''On sites''', when hover scrollbar area, they reappear. AND while pointer is over the track or thumb, it remains visible - does not reduce to an unusable width or disappear completely. As I think it should. But in '''bookmarks library''', the track & thumb disappear, even w/ pointer hovered, unless thumb is grabbed immediately. Even grabbing thumb & move it, then pause a fraction of second (''still gripping'' thumb w/ L mouse button), the thumb & track STILL disappear. As long as you don't release grip (on now invisible thumb / track) they reappear when move pointer. The bookmark library scrollbar behavior is also erratic. How long you have to grab it, when you 1st hover it. If you accidentally move pointer from folder pane, past vertical divider ''into bookmark pane'', then left - back into folder pane, the scrollbar doesn't appear. You have to move pointer well left of scrollbar, then approach it '''from the R''', to make it reappear. It doesn't have any of this behavior in the browser (never has, AFAIK).