Position div at exact cursors location

Question

I am using this color wheel picker, and I'm trying to add a div as the dragger instead of having it embedded in the canvas. I got it working thanks to these answers.

The problem is, the dragger is a bit off from the cursor. The obvious solution would be to just subtract from the draggers left and top position. Like this:

dragger.style.left = (currentX + radiusPlusOffset - 13) + 'px';
dragger.style.top = (currentY + radiusPlusOffset - 13) + 'px';

Another problem comes up when I subtract 13. If you drag the dragger all the way to the right or bottom, it doesn't go all the way. If you drag it all the way to the left or top, it goes passed the canvas's border.

Basically what I'm trying to achieve, is to have the dragger at the cursor pointers exact location, and the draggable shouldn't go passed the canvas's border. How can I achieve that?

JSFiddle

var b = document.body;
var c = document.getElementsByTagName('canvas')[0];
var a = c.getContext('2d');


var wrapper = document.getElementById('wrapper');
var dragger = document.createElement('div');
dragger.id = 'dragger';
wrapper.appendChild(dragger);
wrapper.insertBefore(dragger, c);


document.body.clientWidth; // fix bug in webkit: http://qfox.nl/weblog/218

(function() {

  // Declare constants and variables to help with minification
  // Some of these are inlined (with comments to the side with the actual equation)
  var doc = document;
  doc.c = doc.createElement;
  b.a = b.appendChild;

  var width = c.width = c.height = 400,
    label = b.a(doc.c("p")),
    input = b.a(doc.c("input")),
    imageData = a.createImageData(width, width),
    pixels = imageData.data,
    oneHundred = input.value = input.max = 100,
    circleOffset = 0,
    diameter = width - circleOffset * 2,
    radius = diameter / 2,
    radiusPlusOffset = radius + circleOffset,
    radiusSquared = radius * radius,
    two55 = 255,
    currentY = oneHundred,
    currentX = -currentY,
    wheelPixel = circleOffset * 4 * width + circleOffset * 4;

  // Math helpers
  var math = Math,
    PI = math.PI,
    PI2 = PI * 2,
    sqrt = math.sqrt,
    atan2 = math.atan2;

  // Setup DOM properties
  b.style.textAlign = "center";
  label.style.font = "2em courier";
  input.type = "range";

  // Load color wheel data into memory.
  for (y = input.min = 0; y < width; y++) {
    for (x = 0; x < width; x++) {
      var rx = x - radius,
        ry = y - radius,
        d = rx * rx + ry * ry,
        rgb = hsvToRgb(
          (atan2(ry, rx) + PI) / PI2, // Hue
          sqrt(d) / radius, // Saturation
          1 // Value
        );

      // Print current color, but hide if outside the area of the circle
      pixels[wheelPixel++] = rgb[0];
      pixels[wheelPixel++] = rgb[1];
      pixels[wheelPixel++] = rgb[2];
      pixels[wheelPixel++] = d > radiusSquared ? 0 : two55;
    }
  }
  a.putImageData(imageData, 0, 0);

  // Bind Event Handlers
  input.onchange = redraw;
  dragger.onmousedown = c.onmousedown = doc.onmouseup = function(e) {
    // Unbind mousemove if this is a mouseup event, or bind mousemove if this a mousedown event
    doc.onmousemove = /p/.test(e.type) ? 0 : (redraw(e), redraw);
  }



  // Handle manual calls + mousemove event handler + input change event handler all in one place.
  function redraw(e) {
    // Only process an actual change if it is triggered by the mousemove or mousedown event.
    // Otherwise e.pageX will be undefined, which will cause the result to be NaN, so it will fallback to the current value
    currentX = e.pageX - c.offsetLeft - radiusPlusOffset || currentX;
    currentY = e.pageY - c.offsetTop - radiusPlusOffset || currentY;

    // Scope these locally so the compiler will minify the names.  Will manually remove the 'var' keyword in the minified version.
    var theta = atan2(currentY, currentX),
      d = currentX * currentX + currentY * currentY;

    // If the x/y is not in the circle, find angle between center and mouse point:
    //   Draw a line at that angle from center with the distance of radius
    //   Use that point on the circumference as the draggable location
    if (d > radiusSquared) {
      currentX = radius * math.cos(theta);
      currentY = radius * math.sin(theta);
      theta = atan2(currentY, currentX);
      d = currentX * currentX + currentY * currentY;
    }

    label.textContent = b.style.background = hsvToRgb(
      (theta + PI) / PI2, // Current hue (how many degrees along the circle)
      sqrt(d) / radius, // Current saturation (how close to the middle)
      input.value / oneHundred // Current value (input type="range" slider value)
    )[3];

    dragger.style.left = (~~currentX + radiusPlusOffset - 13) + 'px';
    dragger.style.top = (~~currentY + radiusPlusOffset - 13) + 'px';
    // Reset to color wheel and draw a spot on the current location. 


    // Draw the current spot.
    // I have tried a rectangle, circle, and heart shape.
    /*
    // Rectangle:
    a.fillStyle = '#000';
    a.fillRect(currentX+radiusPlusOffset,currentY+radiusPlusOffset, 6, 6);
    */

    // Circle:
    /*a.beginPath();  
    a.strokeStyle = 'white';
    a.arc(~~currentX+radiusPlusOffset,~~currentY+radiusPlusOffset, 4, 0, PI2);
    a.stroke();*/


    // Heart:
    //a.font = "1em arial";
    //a.fillText("♥", currentX + radiusPlusOffset - 4, currentY + radiusPlusOffset + 4);

  }

  // Created a shorter version of the HSV to RGB conversion function in TinyColor
  // https://github.com/bgrins/TinyColor/blob/master/tinycolor.js
  function hsvToRgb(h, s, v) {
    h *= 6;
    var i = ~~h,
      f = h - i,
      p = v * (1 - s),
      q = v * (1 - f * s),
      t = v * (1 - (1 - f) * s),
      mod = i % 6,
      r = [v, q, p, p, t, v][mod] * two55,
      g = [t, v, v, q, p, p][mod] * two55,
      b = [p, p, t, v, v, q][mod] * two55;

    return [r, g, b, "rgb(" + ~~r + "," + ~~g + "," + ~~b + ")"];
  }

  // Kick everything off
  redraw(0);

  /*
  // Just an idea I had to kick everything off with some changing colors…
  // Probably no way to squeeze this into 1k, but it could probably be a lot smaller than this:
  currentX = currentY = 1;
  var interval = setInterval(function() {
      currentX--;
      currentY*=1.05;
      redraw(0)
  }, 7);
    
  setTimeout(function() {
      clearInterval(interval)
  }, 700)
  */

})();

#c {
  border: 7px solid black;
  border-radius: 50%;
}
#wrapper {
  width: 400px;
  height: 400px;
  position: relative;
  cursor: pointer;
}
#wrapper:active {
  //cursor: none;

}
#dragger {
  width: 8px;
  height: 8px;
  border-radius: 50%;
  display: block;
  position: absolute;
  border: 2px solid black;
}

<div id='wrapper'>
  <canvas id="c"></canvas>
</div>

Answer 1

You're just subtracting in the wrong place.
Instead of subtracting from the elements position, subtract directly from the mouse pointers position.

This code actually moves the element, offsetting it relative to the pointer, and making it appear to be outside the borders of the color picker

dragger.style.left = (~~currentX + radiusPlusOffset - 13) + 'px';
dragger.style.top = (~~currentY + radiusPlusOffset - 13) + 'px';

... which is not what you really want, you want the calculated numbers for the pointer to be exactly center of the dragger element, so you should extract from the pointers position instead, that way the limits of the dragger isn't affected, and it stays within the borders of the color picker

currentX = e.pageX - c.offsetLeft - radiusPlusOffset -13 || currentX;
currentY = e.pageY - c.offsetTop - radiusPlusOffset -13 || currentY;

FIDDLE