This movement isn’t particularly noticeable on a day-to-day basis, but if you stick something onto the phone’s screen as a reference point, you can clearly see the button shift back and forth, ever so slightly.
This is a problem usually associated with old CRT monitors, and happens when a single image is left on-screen for too long, creating a permanent “ghost” on the display.
This problem was actually what led to the creation of screensavers, the original use of which was to change the pixels on the display, preventing any static image being shown for too long.
Although most of us don’t ever think about screen burn-in anymore, it can still be a problem with OLED displays like the one used in the S8.
Smartwatches with OLED screens can suffer from burn-in, too, because their displays are often left on at a high brightness with static elements showing (like parts of the watchface).