Slate SBorder padding

I’m trying to make a box inside another box. The inner box sticks to the top, left and bottom side of the outer box. I’m using SBorder widgets that have their BorderImage properties set to Brushes. Padding is set to 0, but there is a little space between the outer box sides and the inner box. I can’t understand what is causing it and I don’t know how to remove this padding.

Is there an actual Border that is invisible?