Bug in mortar construction

[cmdoug]

This is a continuation from #319. There, an issue with a false flag overlapping edges error was fixed, but there remains a bug with mortar construction. This bug commonly appears when using adaptmesh() on complex geometries with sharp corners. The MWE below triggers the bug (at least on FF v4.16).

real n0 = 128;
real n1 = 32;
real h = 1.0e-4;

// Define borders
border C01(t=0, 1){x=-1; y=1-t; label=1;}
border C02(t=0, 1){x=-1+2*t; y=0; label=2;}
border C03(t=0, 1){x=1; y=2*t; label=3;}
border C04(t=0, 1){x=1-2*t; y=2; label=4;}
border C05(t=0, 1){x=-1; y=2-(1-h)*t; label=5;}
border C06(t=0, 1){x=-1+0.95*t; y=1+h; label=6;}
border C07(t=0, 1){x=-0.05*(1-t); y=1+h*(1-t); label=7;}
border C08(t=0, 1){x=-t; y=1; label=8;}
// Assemble mesh
mesh Th = buildmesh(C01(n1) + C02(n1*2) + C03(n1*2) + C04(n1*2) + C05(n1*(1-h)) + C06(n0*0.95) + C07(n0*sqrt(0.05^2+h^2)) + C08(n0));

/*
fespace Vh(Th, P2);
Vh u;
func f = 1;
varf dot(u, v) = int2d(Th)(dx(u)*dx(v) + dy(u)*dy(v)) + int2d(Th)(f*v) + on(5, u = 0);

matrix A = dot(Vh, Vh);
real[int] b = dot(0, Vh);

u[] = A^-1*b;
plot(u);
*/