I should know better: why is this not a closed shape that I can extrude?

[rpsalas]

I tried all my tricks and can't find the error. If you solve it also please remind me of a reliable technique so I never forget again!

[TrippyLighting · Accepted solution]

When you drag the white line-end point, you'll see that it isn't constrained. An coincident constraint with the other line end will take care of this.

[rpsalas]

Thanks! I was never clear what the white vs. dark points are trying to tell me. That was it!

[dargas22 · Accepted solution]

This one helped me to remember how to combat this issue.
https://www.youtube.com/watch?v=rD2kfmKYMyg