rectangle(@x_spinbox.value, @y_spinbox.value, @width_spinbox.value, @height_spinbox.value) { # a dynamic path is added semi-declaratively inside on_draw block
          fill r: @red_spinbox.value, g: @green_spinbox.value, b: @blue_spinbox.value, a: @alpha_spinbox.value / 100.0
        }