Move the mouse across the screen to move the circle. The program constrains the circle to its box. 

mx = 0
my = 0
easing = 0.05
radius = 24
edge = 100
inner = edge + radius


def setup():
    size(640, 360)
    noStroke()
    ellipseMode(RADIUS)
    rectMode(CORNERS)


def draw():
    global mx, my

    background(51)

    if abs(mouseX - mx) > 0.1:
        mx = mx + (mouseX - mx) * easing
    if abs(mouseY - my) > 0.1:
        my = my + (mouseY - my) * easing
    mx = constrain(mx, inner, width - inner)
    my = constrain(my, inner, height - inner)
    fill(76)
    rect(edge, edge, width - edge, height - edge)
    fill(255)
    ellipse(mx, my, radius, radius)