What is this "shadow" called and what's the purpose? When you slide Chrome browser (or FireFox) to extreme left, right or upwards, you see this shadow (for want of a better word) for a few seconds, as shown




What is it called and what's the purpose ? (Can't Google since I don't know what it is :)

It's on Moto X Play and it's not the jelly scrolling reported on some recent phones
This appears to be called EdgeEffect and is used as a visual indicator when the user over-scrolls. 


See relevant images on Google Images here. 
A book commenting on the glow effect produces.
A question on Stack Overflow.