iPhone 6: battery replacement and display problem
When they replace the battery, they have to completely remove the display. If it is an iPhone 6, there actually isn't any adhesive between the display and the enclosure. It is possible to two screws at the bottom are loose, and when they put the display back on, they will tighten those. I would just mention the display problem when you drop it off for the battery. They should at least be able to tell if it's the screws or something more advanced.