JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="txt" autofocus></textarea>
<div id="suggestions"></div>

CSS

#txt {
  resize:none;
  width:450px;
  height:100px;
  border-radius:5px;
  border:1px solid #eee;
}
#suggestions {
  /* It's better to contain textarea
  and suggestions in a div and make this suggestions view
  absolute positioned */
  box-shadow:0 0 25px rgba(0,0,0,0.05);
  width:450px;
}
#suggestions > div {
  padding:10px;
}
.name-view {
  margin-right:5px;
}
.username-view {
  color: #aaa;
}

JavaScript

function getCursorPosition(el) {
    var start = 0, end = 0, normalizedValue, range, textInputRange, len, endRange;
    if (typeof el.selectionStart == "number" && typeof el.selectionEnd == "number") {
        start = el.selectionStart;
        end = el.selectionEnd;
    } else {
        range = document.selection.createRange();

        if (range && range.parentElement() == el) {
            len = el.value.length;
            normalizedValue = el.value.replace(/\r\n/g, "\n");

            // Create a working TextRange that lives only in the input
            textInputRange = el.createTextRange();
            textInputRange.moveToBookmark(range.getBookmark());

            // Check if the start and end of the selection are at the very end
            // of the input, since moveStart/moveEnd doesn't return what we want
            // in those cases
            endRange = el.createTextRange();
            endRange.collapse(false);

            if (textInputRange.compareEndPoints("StartToEnd", endRange) > -1) {
                start = end = len;
            } else {
                start = -textInputRange.moveStart("character", -len);
                start += normalizedValue.slice(0, start).split("\n").length - 1;

                if (textInputRange.compareEndPoints("EndToEnd", endRange) > -1) {
                    end = len;
                } else {
                    end = -textInputRange.moveEnd("character", -len);
                    end += normalizedValue.slice(0, end).split("\n").length - 1;
                }
            }
        }
    }
    return {
        start: start,
        end: end
    };
}

function setCursorPosition(input, start, end) {
    if (arguments.length < 3) end = start;
    if ("selectionStart" in input) {
        setTimeout(function() {
            input.selectionStart = start;
            input.selectionEnd = end;
        }, 1);
    }
    else if (input.createTextRange) {
        var rng = input.createTextRange();
       ...