Why is the background div running over the edge?

Question

css

why at the bottom of the diva comes another div to the bottom edge (stroke)

Answer 1

Because width auto;
and the element is stretched by width BUT it doesn't take into account the border
calc(100% - 1px)

Answer 2

* {
  box-sizing: border-box;
}

Add what is above. Importers remove

Answer 3

Instead of border-width:1.5px !important; write border-width:2px !important;