scaling behaviour of the standard box texture?

Hi!

So We've been trying to create a custom GUI for our game, and are having trouble with creating textures in the same format as the standard box textures, making sure edges don't get scaled.

The standard box has rounded edges, but no matter the aspect ratio of the actual GUI elements, the edges don't seem to change. For example, a 100x100px box has the same round edges as a 150x540px box.

In our case, we don't have round edges, but a patterned line around the edges. These lines are clearly getting scaled, so I'm wondering how unity handles these textures.

Any idea's?