Legal guarding position

[CoachW]

If A1 has their foot out in front, and then puts their weight on their back foot. B1 then comes up and, the way I picture it, almost straddles A1's front leg, without causing contact. Since A1 had that front leg there first, are they entitled to the space above it because of verticality? Or does that not apply, since you could also say B1 is entitled to that space since they got there without fouling.

Also, but unrelated, how do I quote an earlier post? I haven't been able to figure that out yet.

Thank You.