I believe it's part of the effort to improve sidebars such that (among other things) they are more usable for mobile. Not sure where I read it, but I recall something recently talking about how GNOME wants to improve the mobile experience overall so stuff like gestures and sidebar improvements were going to be coming.

What I did see only at a second glace are those arrows which were added in narrow mode:

As already mentioned. more will probably come in the future