问绘画应用程序在移动设备上不起作用。为什么会这样呢？

EN

<input type="color"  class="js-color-picker  color-picker">
<input type="range" class="js-line-range" min="1" max="72" value="1">
<label class="js-range-value">1</label>Px
<canvas class="js-paint  paint-canvas" width="200" height="200"></canvas>

.paint-canvas {
  border: 1px black solid;
  display: block;
  margin: 1rem;
}

.color-picker {
  margin: 1rem 1rem 0 1rem;
}

const paintCanvas = document.querySelector( '.js-paint' );
const context = paintCanvas.getContext( '2d' );
context.lineCap = 'round';

const colorPicker = document.querySelector( '.js-color-picker');

colorPicker.addEventListener( 'change', event => {
    context.strokeStyle = event.target.value; 
} );

const lineWidthRange = document.querySelector( '.js-line-range' );
const lineWidthLabel = document.querySelector( '.js-range-value' );

lineWidthRange.addEventListener( 'input', event => {
    const width = event.target.value;
    lineWidthLabel.innerHTML = width;
    context.lineWidth = width;
} );

let x = 0, y = 0;
let isMouseDown = false;

const stopDrawing = () => { isMouseDown = false; }
const startDrawing = event => {
    isMouseDown = true;   
   [x, y] = [event.offsetX, event.offsetY];  
}
const drawLine = event => {
    if ( isMouseDown ) {
        const newX = event.offsetX;
        const newY = event.offsetY;
        context.beginPath();
        context.moveTo( x, y );
        context.lineTo( newX, newY );
        context.stroke();
        //[x, y] = [newX, newY];
        x = newX;
        y = newY;
    }
}

paintCanvas.addEventListener( 'mousedown', startDrawing );
paintCanvas.addEventListener( 'mousemove', drawLine );
paintCanvas.addEventListener( 'mouseup', stopDrawing );
paintCanvas.addEventListener( 'mouseout', stopDrawing );

paintCanvas.addEventListener("touchstart", startDrawing);
paintCanvas.addEventListener("touchend", stopDrawing);
paintCanvas.addEventListener("touchcancel", stopDrawing);
paintCanvas.addEventListener("touchmove", drawLine);