The border you see is what we call a "quad". When you're looking at an image in a synth, there are a lot of other images that we can show you from the same position. Some of these we turn on partially (they look a little faded), but if we show all of them - the scene would get too crowded. Instead we offer the quad - just enough information to let you know that something else exists there.
If you wish for it to turn off, just stop moving the mouse. It will fade away after a second. We have an API to turn it permenantly off, but I don't think it's available on the website.