SCREEN 12 works perfectly.
And try this, because it too works perfectly.
SCREEN 13
LINE (0, 0)-(100, 100), , B
CIRCLE (50, 50), 50, , , , 1
The problem is that the circle comes out as an ellipse, if you don't specify an aspect ratio in SCREEN 13. So, if you specify aspect ratio 1, the circle is round, and touches the lines of the box. Otherwise, the circle gets an aspect ratio of about 0.8, instead of defaulting to 1.