I first saw these in 3.5, I have no clue how to fix those as they’re distracting me.
They only appear in edit mode.
Hi Svartberg,
I guess the only solution for you is to buy a bigger screen…hehe kidding 
It’s actually help you to navigating around the object, esp when you zoom-in too much until you cannot see the blank background screen to rotate, so that thing is kinda “the save region” for you to do rotate-scale-move.
It only appear when your object is out of the document area.
This feature was introduced in ZBrush ver3.1 if I’m not mistaken.
Hope its answer your question.
Cheers.
_Revel
Thanks Revel, I must have a bug then - because this happens when my object is way inside my document area.
I don’t see the use of this really, in 3.5 you no longer need to use the blank background to rotate/zoom/pan - just press mmb+rmb instead of lmb.
Those lines appear when the edges of the document (vertical, horizontal or both) extend beyond the canvas window. In this case, if you use the Zoom Document icon to reduce the document height, the lines will disappear.
Sven