Hi, and thanks for this great "little" piece of software with so much punch. SolveSpace is the first and only CAD I've learned to use.

I've got a situation that requires me to force everything to triangle meshes which doesn't seem right. I'm sure this bug is covered by one of the issues in #738, but it may be relevant that every angle is 90 degrees in this model.

System information

• SolveSpace version: 3.1~70bde63c and reproducible on custom built latest master 7d379e7f3b0
• Operating system: Reproduced on Arch Linux and GNU Guix

I've managed to reproduce this on the Arch Linux AUR package and on my GNU Guix system. I don't think it's a packaging issue.

Expected behavior

I've forced NURBS to triangle mesh is on to get that screenshot

Actual behavior

When Force NURBS to triangle mesh off, I'm missing an entire side surface.

Additional information

bug-report-naked-edges-with-only-cuboids-extrude-and-unions.zip

The model is easily produced as follows:

• Make rectangle A
• Extrude it downwards
• Make rectangle B in new sketch using previous sketch workplane or P
• Extrude it upwards
• Make rectangle C in new sketch using B's equivalent of point P (selecting its two connected lines)
• Extrude it upwards
• Make rectangle D in new sketch using P and its two perpendicular lines (or point on the other two cuboids)
• Extrude it right

The extrusion on A is in the opposite direction from B's extrusion. I cannot reproduce this bug if both are extruded in the same direction. The bug depends on the height of D: If D touches just A and B everything is ok. What's very strange is that if D if just a little bit above C, it still works:

Here, D touches C just a tiny bit and force NURBS to triangle mesh is off!