当页面能够滚动时，无法使用鼠标在画布上的精确点绘制矩形

如何解决当页面能够滚动时，无法使用鼠标在画布上的精确点绘制矩形

尽管如果我们在不滚动画布或页面的情况下绘制矩形，代码就可以正常工作。但是，如果我们进行滚动，则从我们开始拖动鼠标的位置开始，矩形就不会出现在屏幕上。

<section className="weather">
  <div className="weather__background" />

  <div className="weather__title">
    <h1>React Weather</h1>
  </div>

  <div className="weather__input">
    <h1>React Weather</h1>
    <input onChange={getLocation} required autoFocus placeholder="Enter a Country..." />
  </div>

  <div className="weather__details">

    <h1>React Weather</h1>
  </div>



</section>

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;

//Mousedown
$(canvas).on('mousedown',function(e) {
    last_mousex = parseInt(e.clientX-canvasx);
    last_mousey = parseInt(e.clientY-canvasy);
    mousedown = true;
});

//Mouseup
$(canvas).on('mouseup',function(e) {
    mousedown = false;
});

//Mousemove
$(canvas).on('mousemove',function(e) {
    mousex = parseInt(e.clientX-canvasx);
    mousey = parseInt(e.clientY-canvasy);
    if(mousedown) {
        ctx.clearRect(0,canvas.width,canvas.height); //clear canvas
        ctx.beginPath();
        var width = mousex-last_mousex;
        var height = mousey-last_mousey;
        ctx.rect(last_mousex,last_mousey,width,height);
        ctx.strokeStyle = 'black';
        ctx.lineWidth = 10;
        ctx.stroke();
    }
    //Output
    $('#output').html('current: '+mousex+','+mousey+'<br/>last: '+last_mousex+','+last_mousey+'<br/>mousedown: '+mousedown);
});

canvas {
    cursor: crosshair;
    border: 1px solid #000000;
}

解决方法

使用this answer计算鼠标位置，效果很好。它考虑了滚动。

function getCursorPosition(canvas,event) {
    const rect = canvas.getBoundingClientRect();
    const x = event.clientX - rect.left;
    const y = event.clientY - rect.top;
    return { x,y };
}

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;

//Mousedown
$(canvas).on('mousedown',function(e) {
    var pos = getCursorPosition(canvas,e);
    last_mousex = pos.x;
    last_mousey = pos.y;
    mousedown = true;
});

//Mouseup
$(canvas).on('mouseup',function(e) {
    mousedown = false;
});

//Mousemove
$(canvas).on('mousemove',e);
    mousex = pos.x;
    mousey = pos.y;
    if(mousedown) {
        ctx.clearRect(0,canvas.width,canvas.height); //clear canvas
        ctx.beginPath();
        var width = mousex-last_mousex;
        var height = mousey-last_mousey;
        ctx.rect(last_mousex,last_mousey,width,height);
        ctx.strokeStyle = 'black';
        ctx.lineWidth = 10;
        ctx.stroke();
    }
    //Output
    $('#output').html('current: '+mousex+','+mousey+'<br/>last: '+last_mousex+','+last_mousey+'<br/>mousedown: '+mousedown);
});

canvas {
    cursor: crosshair;
    border: 1px solid #000000;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas" width="800" height="500"></canvas>
<div id="output"></div>