Boolean Operations on Polygons
Got it. My mistake. I assumed that splitting curves at their horizontal extrema would be enough. But in the case below the green curve ended up above the blue curve in the sweep line buffer. That's because SweepComp is using the end points of the curve (red line) and then of course the green line ends up above the blue one.
I could solve that by also splitting curves at the vertical extremas (orange line)... or by using the left point's control point?