> seems to be impossible to put into an algorithm.

I do understand the conundrum. This is a tricky thing to negotiate with solid/flat objects. If I was drawing this with pen and paper where the line quality had a bit of natural "grunge" in it, the thick line may have tapered to match the thinner line, or the thinner vertical line would have shared the transition. This is easier to envision when drawing more organic character.

Not possible it seems to do with splines and outlines.