Hi Pilou, boolean union does some special work when unioning co-planar planes together that will reform them into one single big plane.

It can also do some special work for the case of unioning together 2 pieces that have the same full underlying surface to combine those together into a single surface as well.

The situation you have there does not fit into either of those special cases, so there won't be any special handling for it to make it any different from the regular generic handling, and in general booleans work by potentially cutting up surfaces into smaller pieces but for gluing things together the generic way is for the pieces to become joined.


> It's because cylinder has been trimed not in perpendicular section?

No, it's because there is special extra work done in the plane/plane case.

- Michael