Why is there a black line following my app when i enter the multitask mode?

[WPCentral Question]

So i've noticed something strange with my lumia 1520, when i enter the multitask mode (where you hold the back key to see all the running apps) there's a black line on both side of the app window the more apps i open there more there is and if i move the app around the lines move with it too? anyone else have the same issue?? could this be a software bug or? thank you in advance for your answers your help is very much appreciated

 

[Suicide Stacey]

I got it too