SFML 2.4.2 Co-ordinate system offset - c++

Question

I am writing a small program using SFML which draws a 5 x 5 rectangle at any point specified by clicking the mouse.
I am using mapPixelToCoords() to map the mouse coordinates to the pixels as recommended in the documentation, however I am still encountering a weird offset between the coordinates and the actual shape which is drawn.
The offset varies on different parts of the window (it is not a constant x,y value).
I would like the rectangle to be drawn precisely under the mouse pointer, how do I go about removing this offset?
This is the code I'm using to draw the rectangle on click:
Screenshot of offset in the window
int main(){
sf::RenderWindow window(sf::VideoMode(800, 720), "SFML window");
sf::Event event;
//Rectangle to be drawn on the screen
sf::RectangleShape someRect;
someRect.setFillColor(sf::Color(0, 0, 0));
someRect.setSize({ 5, 5 });
someRect.setPosition(100, 120);
while (window.isOpen())
{
if (!windowStarted) {
event.type = sf::Event::Closed;
}
while (window.pollEvent(event))
{
// Close window: exit
if (event.type == sf::Event::Closed) {
window.close();
}
if (event.type == sf::Event::MouseButtonPressed) {
someRect.setPosition(window.mapPixelToCoords(sf::Mouse::getPosition(window)));
}
}
window.clear(sf::Color(255, 255, 255));
window.draw(someRect);
window.display();
}
return 0;
}