VFX artist here. It's a great shot, Ken Ralston is a bad-ass.
The shot begins at the foot of the stairs, with the camera on a Steadicam rig. They actually had to paint out a bit of a camera operator that wound up in-frame while the poor camera guys ran around.
At the top of the stairs, they manually speed-ramp the framerate higher, slowing down the footage mid-shot. She is still on the set, and this would have been done in-camera on the day.
The first phase of the effect is the fake mirror. As soon as you start seeing subtle glass artifacts and the frame of the mirror itself, you are seeing something that was not on set. The glass effect comes first, it's just a very slight distortion and an overlay of subtle mirror crud.
The mirror frame and "second Ellie" reaching for the mirror is a separate take, shot on bluescreen. The set could have been just one blue wall, with a prop mirror and the actress, with the "mirrored" part of the mirror prop also painted blue. The camera in the second take started close enough to the mirror that all it saw was blue, and pulled back away from the mirror at the same rate as the camera was moving in take one. We run this take over take one, with the "blue hole" in take two acting as a frame around take one.
As the camera pulls back away from the mirror on take two, we reveal Ellie from behind (surrounded by bluescreen, nothing but her and the mirror on set) and she opens up the prop mirror, finally "hiding" take one from us, and fiddles with the pill bottles. (Don't forget that the mirror had to "reflect" the non-existent bathroom as it swings open, too!)
It's an outstanding effect.
Ken Ralston is a super bad-ass. Other credits as VFX Supe include Forrest Gump, Cast Away, The Rocketeer and Back to the Future.