Created attachment 4779 example screenshot and a very rough pixel-pushing proposal of improvement The new sidepane layout for the bookmarks view has some weird whitespace between the parent items (DEVICES, PLACES, NETWORK) and their children. It looks/seems like there is more whitespace between a heading (eg PLACES) and its first child (eg Home) than between the heading and last child of the previous item (eg DEVICES -> File System). I would suggest using less vertical whitespace below a parent name (eg DEVICES) so the first child is more close to it. This will improve the appearance and also save some precious vertical space.
The space is exactly 6px. But it looks like its not in the middle with some icons (check with selection). Anyway, I agree but there is no easy way to do this with gtk htat i know of.
Not much I can do about this in standard treeviews.