https://cdn.discordapp.com/attachments/692939668160774204/1127557189716955136/RDIC_rounded_corners.mp4
Rounded corners, drawn through X11 draw commands. Why is that share-worthy? Because I want the borders to grow exactly inwards. When X11 draws a line half the line width goes on each side of the line. Imagine the pain.
Anyway, only the arcs for the rounded corners were a real pain the the solution was to now draw arcs at all because they are nigh-uncontrollable if you want pixel-perfect results (i.e. no weird artifacts), so I can recommend overlapping XFillArc drawing calls.
(Oh and thanks to the Nuklear project for their relatively workable example code, though it isn't quite up to my pedantic standards, hence my woes)