My opponent lands on a debris. I fly smack into him. I have Zeb equipped.

I attack my opponent, with our contact point on the debris. Is the attack obstructed?

Gut says yes, because even if the line of attack is zero length, both the start and end points are on the debris cloud.

I don't believe this situation is clearly defined in the rules as written. However, as a tournament organizer / judge, I would rule that yes, it is obstructed, because if they were even the tiniest bit further apart, it would be obstructed.

And this game doesn't tend to shy away from "a zero magnitude thing is still a thing" (e.g. rolling zero defense dice, modifying zero [eye] results), so I wouldn't expect a zero-length line not to count.

I'd say yes. It;s only really a question if the nubs weren't involved in the bump. If they were, no question, the attack's obstructed.