How odd. I watched that debate and the stage set up was with Trump on the left, with a chair there, and Clinton on the right with a chair on her side. The picture and comments were about her speaking to the seated audience on Trump's side of the stage. She walked across to the left to be facing them, appropriately, and was therefore on Trump's side of the stage (just calling it that because of the setup). When the camera was switched to that side of the stage so as to show Clinton face-on it appeared that Trump was behind Clinton facing her back. I don't think he even moved from his chair location. She moved over to be in front of him to address that side of the audience. There was no sneaking up behind her by Trump. Why don't they show the whole stage from the "front" where the moderator was? It puts this stupid comment to rest as a lie.
So sad.