unstable "failed to assemble polygon to trim nurbs surface in uv space"

[vthriller]

System information

• SolveSpace version: 3.1~70bde63c
• Operating system: Gentoo Linux

Actual behavior

...depends on extrusion depth.

Additional information

nurbs-2.slvs.gz

[ruevs]

For cross reference #1390 (comment).

[phkahler]

Some observations: This happens intermittently for all 3 boolean operations (differnce, union, intersection). It also happens if we replace the arc in the first sketch with 3 line segments approximating the angles at the attachment points. It does not have a problem if the final extrusion length does not extend past that large flat surface.

[phkahler]

Update by replacing the arc with 2 lines and adjusting the extrusion to show the issue I get this:

Then dragging the sharp corner around the problem goes away if that triangular part is above or to the right of the area in quesion. I believe this is the same as issue #1367 which I just updated with a picture and some ideas on what's failing.