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@m_magalhaes
Thanks Manuel!

When I fill-in only Inner edges in NgonBase::BuildNgon, then it surprisingly works
I don't know why I tested all combinations except this one
There are still some minor glitches, some inner edges are randomly not hidden, but ngons created are valid for all the cases I have tested.
Random glitches are easily fixable with forcing all inner edges to be hidden.

I will test it more, but this seems to be reliable for now.
NGONS import - read outer edges directly from Pgon::*m_Edge array
NGONS export - with NgonBase::BuildNgon(), filling Inner edges only, and force hide inner edges on quads.

Thanks a lot Manuel,
Your help has been very valuable to me.

Regards,
Viktor.

I just found out it's a GPU issue.
Pixels are not visible on Intel UHD 630.
Mac M1 well.
GeForce works on 4K monitor but not on HD monitor.

So it is a bug.