How can I get the mouse position on an HTML5 canvas when canvas is fullscreen?

Question

The following code works perfectly while the canvas is not fullscreen.

    let canvas = document.querySelector('canvas');
    let ctx = canvas.getContext('2d');

    document.addEventListener('mousemove',e=>{
        let br = canvas.getBoundingClientRect();
        let mouseX = e.clientX - br.left;
        let mouseY = e.clientY - br.top;
    });

However, after canvas.requestFullscreen() variables mouseX, mouseY do no longer represent the mouse's position on the canvas.
Is there any way to get the mouse's position relative to the canvas?