JSFiddle - React, Tailwind, and code Playground

HTML

<canvas id="canvas" width=500 height=500></canvas>

CSS

body {
    background-color: ivory;
    padding:20px;
}
#canvas {
    border:1px solid red;
}
input {
    width:15px;
}

JavaScript

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
function reOffset(){
  var BB=canvas.getBoundingClientRect();
  offsetX=BB.left;
  offsetY=BB.top;        
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }

var isDown=false;
var startX,startY;

var rects=[];
var newRect;

$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});


function handleMouseDown(e){
  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  startX=parseInt(e.clientX-offsetX);
  startY=parseInt(e.clientY-offsetY);

  // Put your mousedown stuff here
  isDown=true;
}

function handleMouseUp(e){
  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  // Put your mouseup stuff here
  isDown=false;

  if(!willOverlap(newRect)){
    rects.push(newRect);
  }
  drawAll();
}

function drawAll(){
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.lineWidth=1;
  ctx.strokeStyle='green';
  for(var i=0;i<rects.length;i++){
    var r=rects[i];
    ctx.strokeRect(r.left,r.top,r.right-r.left,r.bottom-r.top);
  }
}

function handleMouseOut(e){
  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  // Put your mouseOut stuff here
  isDown=false;
}

function handleMouseMove(e){
  if(!isDown){return;}
  // tell the browser we're handling this event
  e.preventDefault();
  e.stopPropagation();

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  newRect={
    left:Math.min(startX,mouseX),
   ...