JSFiddle - React, Tailwind, and code Playground

HTML

<div id="fake_area"><span></span></div>
<div id="caret"></div>
<textarea id="textarea">test</textarea>

CSS

textarea, #fake_area {
    position: absolute;
    margin: 0;
    padding: 0;
    height: 400px;
    width: 600px;
    font-size: 16px;
    font: 16px "Courier New", Courier, monospace;
    white-space: pre;
    top: 0;
    left: 0;
    resize: none;
	outline: 0;
	border: 1px solid orange;
	overflow: hidden;
    word-break: break-word;
    padding: 5px;
    	-webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    -ms-box-sizing: border-box;
    box-sizing: border-box;
}
#fake_area {
    /* hide */
    opacity: 0;
}
#caret {
    width: 1px;
    height: 18px;
    position: absolute;
    background: #f00;
    z-index: 100;
}

JavaScript

var fake_area = document.getElementById("fake_area").firstChild;
var fake_caret = document.getElementById("caret");
var real_area = document.getElementById("textarea");

setInterval(function () { doStuff(); }, 10);

function doStuff() {
        var newHTML = real_area.value.substring(0, getCaretPosition(real_area)).replace(/\n$/, '\n\u0001');
    if (fake_area.innerHTML != newHTML)
    {
        fake_area.innerHTML = newHTML;
        setCaretXY(fake_area, real_area, fake_caret,                         getPos("textarea"));
    }
}

    
    function getCaretPosition(el) {
        if (el.selectionStart) return el.selectionStart;
        else if (document.selection) {
            //el.focus();
            var r = document.selection.createRange();
            if (r == null) return 0;
            
            var re = el.createTextRange(), rc = re.duplicate();
            re.moveToBookmark(r.getBookmark());
            rc.setEndPoint('EndToStart', re);
    
            return rc.text.length;
        }
        return 0;
    }

    function setCaretXY(elem, real_element, caret, offset) {
    	var rects = elem.getClientRects();
    	var lastRect = rects[rects.length - 1];
    	
        var x = lastRect.left + lastRect.width - offset[0] + document.body.scrollLeft,
    		y = lastRect.top - real_element.scrollTop - offset[1] + document.body.scrollTop;
    	
//    	caret.style.cssText = "top: " + y + "px; left: " + x + "px";
        caret.style.top=y+"px";
        caret.style.left=x+"px";
    	//console.log(x, y, offset);
    }

    function getPos(e) {
    	e = document.getElementById(e);
	    var x = 0;
	    var y = 0;
	    while (e.offsetParent !== null){
		    x += e.offsetLeft;
		    y += e.offsetTop;
		    e = e.offsetParent;
	    }
    	return [x, y];
    }