Most of this week's difficulty lived inside one single rule, and what made it worth writing about isn't any one fix — it's that I didn't actually understand the problem until I'd already "solved" it three times.
The rule sounds small: if you click through a point on your own path that would carry you past your color's still-unreached endpoint, the path should stop right there and count as done. The first version I built didn't do either half of that. Clicking straight through just kept going — the path walked right past the endpoint like it wasn't special at all, and on top of that, nothing registered as complete. So the very first attempt wasn't off by one small detail, it just didn't implement the rule at all, even though I'd been sure it did.
I fixed the "doesn't stop" part, and that's when things got interesting instead of just wrong. Now, clicking through the endpoint did stop — but it stopped the way an illegal move stops: the little rejection motion played, the same one you get from hitting a wall or crossing your own path somewhere you shouldn't. Which is backwards. Reaching your own endpoint isn't a mistake, it's the point of the whole color. I'd fixed the symptom by routing it through the wrong branch of logic entirely — treating "stop here because you're done" as if it were "stop here because that's illegal."
