Might want to make it extra clear that you’re looking strictly for land borders? Today’s country, Denmark, has a bridge that connects it to its neighbor, Sweden. But Sweden was not what Neighborle was looking for as a valid answer.
It seems like Greenland is treated as a separate territory, that's why it's not red and can be selected in the dropdown. Not sure if it knows about Canada - Greenland border though... I guess we'll find out when it shows up
The drainage boundaries AFAICT. On one side of the line any rainfall flows downhill to joins a river that ends up in the North Sea, on the other side of the line it goes downhill to any other body of water.
Example: https://upload.wikimedia.org/wikipedia/commons/b/b8/North_se...