Default Shield Screen

Hello

A strange thing happen (maybe since the beginning ?) on the shield's screen but to reproduce it , it's not easy...
This thing is a vertical black line on the left side of the screen that appears one second and disappear...i can often reproduce it with a dark background and by scroll down and up the 'tool bar' (where there are the clock, wifi, battery...)

Really strange...

Bye !

Update : I do not reproduce the problem today...