Square stroke cap results in same behavior as butt stroke cap bug

tiled_lines.rs

use nannou::prelude::*;

/// Generative Artistry Blog, Tiled Lines
fn main() {
    nannou::app(model)
        .run();
}

struct Model {

}

fn model(app: &App) -> Model {
    let _window = app.new_window()
        .size(720, 720)
        .view(view)
        .build()
        .unwrap();

    Model {  }
}

fn view(app: &App, _model: &Model, frame: Frame){
    if app.elapsed_frames() == 1 {
        let draw = app.draw();
        draw.background().color(WHITE);

        draw.rect()
            .wh(app.window_rect().wh())
            .color(WHITE);

        let left = app.window_rect().left() as i32;
        let right = app.window_rect().right() as i32;
        let top = app.window_rect().top() as i32;
        let bottom = app.window_rect().bottom() as i32;
        let step = 60;

        for i in (left..right).step_by(step) {
            for j in (bottom..top).step_by(step) {
                draw_line(&draw, i as f32, j as f32, step as f32, step as f32);
            }
        }

        draw.to_frame(app, &frame).unwrap();
    }
}

fn draw_line(draw: &Draw, x: f32, y: f32, w: f32, h: f32) {
    let left_to_right = random_f32() >= 0.5;

    if left_to_right {
        let start_point = pt2(x, y);
        let end_point = pt2(x+w, y+h);

        draw.line()
            .start(start_point)
            .end(end_point)
            // .caps_square()
            // .caps_round()
            .caps_butt()
            .weight(10.0)
            .color(PINK);

        draw.rect()
            .xy(start_point)
            .w_h(2.0, 2.0)
            .color(BLACK);

        draw.rect()
            .xy(end_point)
            .w_h(2.0, 2.0)
            .color(BLACK);
    } else {
        let start_point = pt2(x+w, y);
        let end_point = pt2(x, y+h);

        draw.line()
            .start(start_point)
            .end(end_point)
            // .caps_square()
            // .caps_round()
            .caps_butt()
            .weight(10.0)
            .color(PINK);

        draw.rect()
            .xy(start_point)
            .w_h(2.0, 2.0)
            .color(BLACK);

        draw.rect()
            .xy(end_point)
            .w_h(2.0, 2.0)
            .color(BLACK);
    }
}