I was helping someone set up their brand new (and stunningly beautiful) Dell XPS 15. When, lo and behold, a mysterious blue box appeared every time I moved a Window.
Finally, as I watched a coworker drag a window, I noticed a program briefly appear as the window as being dragged, then close itself once I stopped.
It turns out the XPS 15 came with a program called “Dell PremierColor” which has an ancillary function of making it “easier” to snap windows.
To disable this, first open the Dell PremierColor app (yes, PremierColor is one word…)