On this play, one foot out of the batter's box equals being out of the box. Whether intentional or not, or whether the batter was looking the other way or oblivious to what was going on behind her doesn't matter.
This is batter interference. Dead ball, batter out, any runners return to their bases.
When the catcher is making a play to first, second or third base the batter's box offers some protection to the batter against an interference call, but that protection is not absolute.
Immediately following a pitch, the rules recognize the fact that the batter is precisely where the rules require her to be- in the batter's box and in a position to receive a pitch. She isn't expected to go "poof" and vanish the instant the catcher receives the ball.
As long as the batter remains in her position, and within the batter's box, the catcher is obligated to create her own throwing lane and manuveur around the batter. If the batter "stays put" and the throw hits her or the catcher bumps into her it is NOT interference.
If that would have been the case on this play, then the ball would remain live and any runners would have been awarded two bases when the throw went out of play. But that wasn't the case! Since the batter was out of the box, she has lost any protection it might have provided. Once the batter is out of the box, she is obligated to not interfere with the catcher or the throw- no matter what throwing lane the catcher takes and no matter if she meant to interfere or not.
Even when the batter stays inside the box, if she moves out of her initial "starting position" of receiving the pitch, she can still be guilty of interference. If she steps backward, forward, moves around in the box, leans out over the plate, takes a practice swing with the bat or otherwise abandons her initial spot, she could still be called for interference despite still being in the box. "Intent" is not relevant. The batter can accidentally interfere.
On these plays, the batter's best option is to "make like a statue" and hold her ground in whatever spot she started out. If she does that, then she should not be guilty of interference in any circumstance.
One more wrinkle on being in the batter's box: If there is a runner advancing to home plate, such as on a steal, bunt, wild pitch or passed ball, then the batter's obligation changes. When a runner is advancing to home, the batter must vacate ANY area needed by the defense to complete the play. In this case, the batter's box offers ZERO protection against an interference call!
This is a different scenario than being in the box immediately following a pitch where the catcher is throwing to a base, it is covered by a completely different rule and the batter has a completely different obligation to get out of the way.
