It may be hard to tell, but in the image I have uploaded there seems to be a nice white edge of the rectangle. I am wanting the same look, but not sure how to go about it. Its more noticeable in the second image
If you sample the white bar, it's not pure white. I was able to recreate that effect using 2 instances of CC Light Sweep. You could easily sweep the effect across the border, if you wanted with couple of keyframes and an expression picking the Center from one effect to the other with a bit of an offset.
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.