The previously-rendered overlay appears on the incoming edge of the screen while scrolling after switching from one overlay to another. Here I am scrolling down-right and you can see the colors on the bottom and right edge. The incoming edge displays the colors from the temperature overlay, which is the last overlay I was using.
Scroll around the screen with an overlay on (the temperature overlay is most visible). Switch to a different overlay, then scroll back over the same area again. For extra fun, toggle between several overlays as you scroll across the first time.