Rounded scrollbars in expanded state
The current expanded state for scrollbars looks a bit heavy, and disconnected from the much more lightweight default styling. Making the trough visuals not go edge to edge, and essentially just "scaling up" the rounded scrollbar instead could be a nice way to make them fit in better.
Functionally this would not change much, because even though the scrollbar is visually a few pixels narrower, the actual click target could be a few pixels larger on both sides.
Mockups here: https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/scrollbars/round-scrollbars.png
cc @jimmac