What is that black border around the edges of the metal base of a laptop keyboard?

On literally every notebook keyboard I've ever taken apart for cleaning and/or repair, I've seen this black border on the front side of the metal plate, just under the membrane layer. Quite difficult to scrape, it comes off fairly easily with acetone. On the back side there is no such edge. (source: YouTube video ) I have always tried to preserve it when wiping, but it is hard enough because there is plenty of key hooks on this side. So now I have two questions: What is the reason for this border? Is it just an adhesive for the membrane layer or does it have some other, say, insulating purpose

Question by cher-nov on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.