compact quick-access index / 2d accordion / rolodex-style

I find that as the number of root pages in a zim repo increases, the utility of the index greatly decreases (beyond child-and-parent navigation), in no small part because (1) my distro has removed the single-line scroll arrows, and (2) dragging is so imprecise that moving even a single pixel often jumps far beyond where you are trying to navigate.

Inspired by how smartphones and old rolodexes manage the high number of entries, I think that it would solve the above problem, but also greatly accelerate manual index navigation to have these collapsible, view-only-one, index points.

Caveats:
* it does use more of the already-precious vertical space in the index panel
* as presented, this probably wouldn't help locales with non-latin character sets
* if "on by default" it would probably serve only to confuse newcomers, and make the interface less friendly

(mockup attached)