Retina Display Issue with SFML on OS X 10.9

Correct.png

Incorrect.png

main.cpp

#include <SFML/Graphics.hpp>
#include <SFML/System.hpp>

int main()
{
	// Create the window
	sf::RenderWindow window(sf::VideoMode(640, 480), "Demo App", sf::Style::Titlebar | sf::Style::Close | sf::Style::Resize);

	// Create a simple shape
	sf::RectangleShape rectangle(sf::Vector2f(640,480));
	rectangle.setFillColor(sf::Color::Red); // You will see red if the shader doesn't work

	// Load the shader
	sf::Shader shader;
	shader.loadFromFile("shader.frag", sf::Shader::Fragment);
	shader.setParameter("window_width", 640.f);
	shader.setParameter("window_height", 480.f);

	while (window.isOpen())
	{
		sf::Event event;
		while (window.pollEvent(event))
		{
			if (event.type == sf::Event::KeyPressed) {
				switch(event.key.code) {
				case sf::Keyboard::Escape:
					window.close();
				default:
					break;
				}
			}

			if (event.type == sf::Event::Resized)
			{
				shader.setParameter("window_width", event.size.width);
				shader.setParameter("window_height", event.size.height);
			}

			if (event.type == sf::Event::Closed)
				window.close();
		}

		window.clear();
		window.draw(rectangle, &shader);
		window.display();
	}

	return 0;
}

shader.frag

uniform float window_width;
uniform float window_height;

void main()
{
	// Calculate the normalized position on the screen.
	// (0,0) is bottom left
	// (1,1) is top right
	vec2 pos = vec2(gl_FragCoord.x / window_width, gl_FragCoord.y / window_height);

	// Map the red component to the x value and the green component to the y value
    gl_FragColor = vec4(pos.x, pos.y, 0.0, 1.0);
}