The current method of creating closed polygons by cutting closed surfaces only works for simple cases (e.g., not too many line segments, polygons not containing each other).

vtkContourTriangulator implements a more sophisticated method (although significantly slower than the current, fragile implementation). It seem to provide reasonable output for reasonable input. There are still issues in case of extreme cases, such as cutting a surface generated by thresholding.

Unfortunately, sometimes the filter hangs indefinitely in
VTK\Filters\General\vtkContourTriangulator.cxx line 2361 when it is used on extremely complicated input. Getting incorrect output for an unreasonably complex input would be acceptable, but causing the application to hand is unacceptable. I wrote to David Gobbi on March 12, 2019, he acknowledged the issue, but did not have time to investigate.

If the hang is fixed then then we can switch to the vtkContourTriangulator filter by merging this branch:

https://github.com/lassoan/Slicer/tree/segmentation-closed-surface-in-slice-view