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However, after canvas.requestFullscreen() variables mouseX, mouseY do no longer represent the mouse's position on the canvas.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?

Answer 1

After a long time I finally found a solution.

var FULLSCREEN = false;
document.onfullscreenchange = () => {FULLSCREEN = !FULLSCREEN};
// Keep Track of when Screen 

var mouseX=0,mouseY=0;

function map(v,n1,n2,m1,m2){
    return (v-n1)/(n2-n1)*(m2-m1)+m1;
}

doucment.addEventListener('mousedown',e=>{
        var x,y;
        var element = e.target;
        let br = element.getBoundingClientRect();
        if(FULLSCREEN){
            let ratio = window.innerHeight/canvas.height;
            let offset = (window.innerWidth-(canvas.width*ratio))/2;
            x = map(e.clientX-br.left-offset,0,canvas.width*ratio,0,element.width);
            y = map(e.clientY-br.top,0,canvas.height*ratio,0,element.height);
        } else {
            x = e.clientX - br.left;
            y = e.clientY - br.top;
        }
        mouseX = x;
        mouseY = y;
});

// Unfortunately this only works if the element is touching the top and bottom of the screen
// In other words, the ratio between the width and height of your screen must 
// be greater that the ratio of width to height for your element