ScrollBar arrows are rendering with unexpected antialising

With the following css code I would hope to get a 4 x 4 pixel square shape for the decrement arrow. I do get the square, but 2 of it's edges are unexpectedly blurred/antialiased. See screenshot

.scroll-bar:vertical > .decrement-button > .decrement-arrow {
    -fx-padding: 2px;
    -fx-shape: "M0 0 L4 0 L4 4 L0 4";
}