Union results are unexpected.

[BigZebra101010]

Here is the sample code:

void TestMerge()
{
    Paths64 subject;
    Clipper2Lib::Paths64 solution;
    SvgWriter svg;
    Clipper64 c;

    c.PreserveCollinear = false;

    subject.push_back(MakePath({ 6555320,12600780 , 6553540,12600780 , 6553540,12599680 , 6553920,12599680 , 6553920,12600060 , 6552580,12600060 , 6552580,12599680 , 6552960,12599680 , 6552960,12603400 , 6551560,12603400 , 6551560,12602560 , 6551940,12602560 , 6551940,12602940 , 6550760,12602940 , 6550760,12600900 , 6551940,12600900 , 6551940,12601280 , 6551560,12601280 , 6551560,12598660 , 6555320,12598660 }));
    subject.push_back(MakePath({ 6588180,12600380 , 6587560,12600380 , 6587560,12600000 , 6587940,12600000 , 6587940,12603400 , 6586500,12603400 , 6586500,12598660 , 6588180,12598660 }));
    subject.push_back(MakePath({ 6592440,12599440 , 6592060,12599440 , 6592060,12599060 , 6593800,12599060 , 6593800,12600980 , 6593420,12600980 , 6593420,12600600 , 6594120,12600600 , 6594120,12602940 , 6593420,12602940 , 6593420,12602560 , 6593800,12602560 , 6593800,12603400 , 6592400,12603400 , 6592400,12600380 , 6592780,12600380 , 6592780,12600760 , 6591680,12600760 , 6591680,12600080 , 6592060,12600080 , 6592060,12600460 , 6589620,12600460 , 6589620,12598780 , 6592440,12598780 }));
    subject.push_back(MakePath({ 6597560,12600820 , 6596940,12600820 , 6596940,12600440 , 6597320,12600440 , 6597320,12603400 , 6595800,12603400 , 6595800,12598660 , 6597560,12598660 }));
    subject.push_back(MakePath({ 6591880,12601020 , 6591500,12601020 , 6591500,12600640 , 6592520,12600640 , 6592520,12603180 , 6590700,12603180 , 6590700,12601840 , 6591080,12601840 , 6591080,12602220 , 6589640,12602220 , 6589640,12600340 , 6591880,12600340 }));
    subject.push_back(MakePath({ 6582160,12597760 , 6581780,12597760 , 6581780,12597380 , 6602060,12597380 , 6602060,12598780 , 6581780,12598780 , 6581780,12598400 , 6582160,12598400 , 6582160,12598860 , 6545640,12598860 , 6545640,12597360 , 6582160,12597360 }));

    c.AddSubject(subject);

    c.Execute(ClipType::Union, FillRule::Positive, solution);

    SvgAddSolution(svg, solution, FillRule::NonZero, false);
    std::string filename = "my_poly.svg";
    SvgSaveToFile(svg, filename, 3000, 1000, 10);
    System(filename);
}

the two Collinear Edges are not merged.

[AngusJohnson]

Amend the following:

Clipper2/CPP/Clipper2Lib/src/clipper.engine.cpp

Lines 2582 to 2588 in 3867aab

	if (horz.outrec && horz.outrec != currHorzOutrec)
	{
	currHorzOutrec = horz.outrec;
	//nb: The outrec containining the op returned by IntersectEdges
	//above may no longer be associated with horzEdge.
	AddTrialHorzJoin(GetLastOp(horz));
	}

to:

        if (horz.outrec)
        {
          //nb: The outrec containining the op returned by IntersectEdges
          //above may no longer be associated with horzEdge.
          AddTrialHorzJoin(GetLastOp(horz));
        }

[AngusJohnson]

Hopefully fixed now.