Unexpected result from Union of Paths64

[egarzago]

While using RectClip, I encountered an odd result and traced it down to a union of the clipped paths. In some instances, the union of several subjects produces a zero-width bridge on the resulting polygon. It seems that performing a second union directly after the result of the first one solves the issue. Here's a minimal reproduction of the issue:

#include <clipper2/clipper.h>
#include <iostream>

namespace
{
    void print_paths(const Clipper2Lib::Paths64& paths)
    {
        for (const auto& path : paths)
        {
            std::cout << "Path with " << path.size() << " points: ";
            for (const auto& pt : path)
            {
                std::cout << "(" << pt.x << "," << pt.y << ") ";
            }
            std::cout << "\n";
        }
    }
}  // namespace

int main()
{
    Clipper2Lib::Paths64 subject;

    subject.push_back(
        {{420, -270}, {500, 0}, {470, 100}, {0, 100}, {0, -483}, {207, -454}}
    );
    subject.push_back(
        {{0, 100}, {370, 100}, {400, 0}, {336, -216}, {166, -363}, {0, -386}}
    );
    subject.push_back(
        {{252, -162}, {300, 0}, {270, 100}, {0, 100}, {0, -289}, {124, -272}}
    );
    subject.push_back(
        {{0, 100}, {170, 100}, {200, 0}, {168, -108}, {83, -181}, {0, -192}}
    );

    auto first_union = Clipper2Lib::Union(
        subject,
        Clipper2Lib::FillRule::NonZero
    );

    std::cout << "First union result has " << first_union.size() << " paths and is: \n";
    print_paths(first_union);

    std::cout << "Performing second union on the first union result...\n";
    auto second_union = Clipper2Lib::Union(
        first_union,
        Clipper2Lib::FillRule::NonZero
    );
    std::cout << "Second union result has " << second_union.size() << " paths and is: \n";
    print_paths(second_union);

    return 0;
}

First union result has 1 paths and is: 
Path with 21 points: (200,0) (168,-108) (83,-181) (0,-192) (0,-289) (0,-483) (207,-454) (420,-270) (500,0) (470,100) (370,100) (400,0) (336,-216) (166,-363) (0,-386) (0,-289) (124,-272) (252,-162) (300,0) (270,100) (170,100) 
Performing second union on the first union result...
Second union result has 2 paths and is: 
Path with 10 points: (0,-289) (124,-272) (252,-162) (300,0) (270,100) (170,100) (200,0) (168,-108) (83,-181) (0,-192) 
Path with 10 points: (207,-454) (420,-270) (500,0) (470,100) (370,100) (400,0) (336,-216) (166,-363) (0,-386) (0,-483) 

These are the original Paths:

This is the result after the first Union (single Path64 output with a zero-width bridge, not sure if that counts as a self-intersection for Clipper2):

And this is the result after the second union (which is the result I expected from the first one):

Using Windows MSVC 19.44 and clipper2/2.0.1 from conancenter https://conan.io/center/recipes/clipper2

Thank you for all the great work on Clipper2!