Union result goes around twice.

[Mitsuhiko-Matsukawa]

boost1.81.0
Visual Studio 2019. x64 debug

void Test()
{
    namespace bg = boost::geometry;
    using point = bg::model::point<double, 2, bg::cs::cartesian>;
    using ring = bg::model::ring<point>;
    using multi_polygon = bg::model::multi_polygon<bg::model::polygon<point>>;
    multi_polygon input0;
    ring input1;

    constexpr double ng_y = -115.4;
    //constexpr double ng_y = -115.5;

    input0.resize(2);
    // green
    input0[0].outer().assign({
        {-63,-88},
        {0,ng_y},
        {0,-124},
        {-63,-124},
        {-63,-88}
    });
    // blue
    input0[1].outer().assign({
        {0,-150},
        {0,-124},
        {13,-124},
        {13,-121},
        {0,ng_y},
        {0,-88},
        {40,-88},
        {40,-150},
        {0,-150}
    });
    // red
    input1.assign({
        {0,-88},
        {0,ng_y},
        {-10,-88},
        {0,-88}
    });

    assert(bg::is_valid(input0));
    assert(bg::is_valid(input1));
    multi_polygon output;
    bg::union_(input0, input1, output);

    for (size_t i = 0; i < output.size(); ++i) {
        std::cout << "====polygon " << i << std::endl;
        for (auto& point : output[i].outer()) {
            std::cout << point.get<0>() << "," << point.get<1>() << std::endl;
        }
    }

    auto area_in = bg::area(input0) + bg::area(input1);
    auto area_out = bg::area(output);
    assert(area_in >= area_out);
}

result

====polygon 0
0,-115.4
0,-124
-63,-124
-63,-88
-4.35207e-13,-115.4
0,-115.4
0,-124
-63,-124
-63,-88
0,-115.4
====polygon 1
0,-115.4
-10,-88
0,-88
40,-88
40,-150
0,-150
0,-124
13,-124
13,-121
-4.35207e-13,-115.4
0,-115.4
0,-88
40,-88
40,-150
0,-150
0,-124
13,-124
13,-121
0,-115.4

If ng_y is not -111.4 but -111.5, it works well.

[barendgehrels]

Thanks for the report. I can reproduce it on Linux / clang

[barendgehrels]

Related to #1108

[barendgehrels]

Fixed by mentioned PR