Show a caret in a custom textarea without displaying its text

Question

I have a custom textarea. In this example, it makes the letters red or green, randomly.

var mydiv = document.getElementById('mydiv'),
    myta = document.getElementById('myta');
function updateDiv() {
  var fc;
  while (fc = mydiv.firstChild) mydiv.removeChild(fc);
  for (var i = 0; i < myta.value.length; i++) {
    var span = document.createElement('span');
    span.className = Math.random() < 0.5 ? 'green' : 'red';
    span.appendChild(document.createTextNode(myta.value[i]));
    mydiv.appendChild(span);
  }
};
myta.addEventListener('input', updateDiv);

body { position: relative }
div, textarea {
  -webkit-text-size-adjust: none;
  width: 100%;
  white-space: pre-wrap;
  word-wrap: break-word;
  overflow-wrap: break-word;
  font: 1rem sans-serif;
  padding: 2px;
  margin: 0;
  border-radius: 0;
  border: 1px solid #000;
  resize: none;
}
textarea {
  position: absolute;
  top: 0;
  color: transparent;
  background: transparent;
}
.red { color: #f00 }
.green { color: #0f0 }

<div id="mydiv"></div>
<textarea id="myta" autofocus=""></textarea>

There's an output div with a textarea over it. So the textarea doesn't cover up any of the colorful things below it, its color and background are set to transparent. Everything works here, except that the caret (the flashing cursor provided by the user agent) is transparent.

Is there a way to show the caret without making the textarea's text visible?

If I make the div above the textarea instead and give it pointer-events: none, the textarea is still visible underneath. This arrangements also makes smooth scrolling difficult, so it doesn't work for me.

Answer 1

Just insert your own caret!

function blink() {
  document.getElementById('caret').hidden ^= 1;
  blinkTimeout = setTimeout(blink, 500);
}
var mydiv = document.getElementById('mydiv'),
    myta = document.getElementById('myta'),
    blinkTimeout = setTimeout(blink, 500),
    lastSelectionStart = 0,
    lastSelectionEnd = 0,
    whichSelection = true;
function updateDiv() {
  var fc;
  while (fc = mydiv.firstChild) mydiv.removeChild(fc);
  if (myta.selectionStart != lastSelectionStart) {
    lastSelectionStart = myta.selectionStart;
    whichSelection = false;
  }
  if (myta.selectionEnd != lastSelectionEnd) {
    lastSelectionEnd = myta.selectionEnd;
    whichSelection = true;
  }
  var cursorPos = whichSelection ? myta.selectionEnd : myta.selectionStart;
  for (var i = 0; i < myta.value.length; i++) {
    if (i == cursorPos) {
      var caret = document.createElement('span');
      caret.id = 'caret';
      caret.appendChild(document.createTextNode('\xA0'));
      mydiv.appendChild(caret);
      clearTimeout(blinkTimeout);
      blinkTimeout = setTimeout(blink, 500);
    }
    var span = document.createElement('span');
    span.className = Math.random() < 0.5 ? 'green' : 'red';
    span.appendChild(document.createTextNode(myta.value[i]));
    mydiv.appendChild(span);
  }
  if (myta.value.length == cursorPos) {
    var caret = document.createElement('span');
    caret.id = 'caret';
    caret.appendChild(document.createTextNode('\xA0'));
    mydiv.appendChild(caret);
    clearTimeout(blinkTimeout);
    blinkTimeout = setTimeout(blink, 500);
  }
};
myta.addEventListener('input', updateDiv);
myta.addEventListener('focus', updateDiv);
myta.addEventListener('mousedown', function() {
  setTimeout(updateDiv, 0);
});
myta.addEventListener('keydown', function() {
  setTimeout(updateDiv, 0);
});
myta.addEventListener('blur', function() {
  document.getElementById('caret').hidden = true;
  clearTimeout(blinkTimeout);
});

body { position: relative }
div, textarea {
  -webkit-text-size-adjust: none;
  width: 100%;
  white-space: pre-wrap;
  word-wrap: break-word;
  overflow-wrap: break-word;
  font: 1rem sans-serif;
  padding: 2px;
  margin: 0;
  border-radius: 0;
  border: 1px solid #000;
  resize: none;
}
textarea {
  position: absolute;
  top: 0;
  color: transparent;
  background: transparent;
}
.red { color: #f00 }
.green { color: #0f0 }
#caret {
  display: inline-block;
  position: absolute;
  width: 1px;
  background: #000;
}
#caret[hidden] { display: none }

<div id="mydiv"><span id="caret">&nbsp;</span></div>
<textarea id="myta" autofocus=""></textarea>

I have here a <span> #caret inserted into the div which blinks every 500ms by toggling its hidden attribute using JS. To replicate browser behavior, I had to detect whether it was the selectionStart or the selectionEnd which the caret was actually at, and make it remain solid while text was being input.

This is a bit harder to achieve when the spans aren't of fixed length or are nested, but it's easier than fiddling with contentEditable with a more complex highlighter. This function will insert the caret in the right spot:

function insertNodeAtPosition(node, refNode, pos) {
    if (typeof(refNode.nodeValue) == 'string') refNode.parentNode.insertBefore(node, refNode.splitText(pos));
    else {
        for (var i = 0; i < refNode.childNodes.length; i++) {
            var chNode = refNode.childNodes[i];
            if (chNode.textContent.length <= pos && i != refNode.childNodes.length - 1) pos -= chNode.textContent.length;
            else return insertNodeAtPosition(node, chNode, pos);
        }
    }
}

Usage (where i is the position to insert it):

var caret = document.createElement('span');
caret.id = 'caret';
caret.appendChild(document.createTextNode('\xA0'));
insertNodeAtPosition(caret, mydiv, i);
clearTimeout(blinkTimeout);
blinkTimeout = setTimeout(blink, 500);