One tiny thing I don't see many UI libraries do when loaded with rounded corners: fix nested rounded boxes by adding the size of the gap between the outer and inner boxes to the border radius. Otherwise, you end up with the slightly off-kilter appearance of the snackbar component for example. [0] Chrome recently added this two-up pane layout for tabs, and it has this exact issue as well. [1]

[0] https://www.chainlift.io/components/snackbar

[1] https://i.imgur.com/uPuTtNb.png