What keeps the glass attached to the screen?

I recently bought a second-hand 5S with a rear camera that wouldn't focus. I bought a replacement camera and started to open the phone with a suction cup. As I pulled, the glass separated completely from the display assembly along the right-hand side of the phone. I immediately stopped applying pressure, and it went back to normal. At this point I aborted the repair. Upon further inspection, I noticed that the upper right-hand corner of the glass has about a half-millimetre of play when I press on it.

The phone works perfectly well, and the glass doesn't seem like it's in any danger of falling out. However, I later did manage to replace the camera and noticed that a lot of dust had made it into the phone in that corner where the glass is not attached. I suspect that this contributed to the original camera failure.

There was already one bit of dust between the glass and the display, and after seeing the camera situation I'm worried about more dust getting in and ruining things. Has anyone else encountered this sort of glass-screen separation? What can I do about it short of replacing the entire assembly?