gradient_plot() not working when one component of gradient is always zero

[dtkaplan]

A work-around is to add a trivial non-zero component, e.g.

gradient_plot(constraint(x, y)+0.001*x ~ x + y, domain(x=c(0,1), y=c(0,1)))

But this should be fixed. Presumably, the zero component is making some matrix of values into a scalar.