JSFiddle - React, Tailwind, and code Playground

HTML

<div id="text">
    Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
</div>

<button onclick="alert(getDisplayedText())">What text is visible right now?</button>

CSS

#text
{
    width: 200px;
    height: 100px;
    overflow-y: scroll;
}

JavaScript

window.onload = function()
{
    var text = document.getElementById("text");
    for (var i = 0; i < text.childNodes.length; i++)
    {
        var child = text.childNodes[i];
        if (child.nodeType == 3 && child.nodeValue.length > 1)
        {
            // Each character should be a separate text node
            var frag = document.createDocumentFragment();
            for (var j = 0; j < child.nodeValue.length; j++)
                frag.appendChild(document.createTextNode(child.nodeValue[j]));
            i += frag.childNodes.length - 1;
            text.replaceChild(frag, child);
        }
    }
};

function getDisplayedText()
{
    var textRect = document.getElementById("text").getBoundingClientRect();
    var range = document.createRange();
    range.selectNodeContents(document.getElementById("text"));

    // Move range start until is in the visible area
    while (range.startOffset < range.endOffset)
    {
        var rangeRect = range.getBoundingClientRect();
        if (rangeRect.top >= textRect.top)
            break;

        range.setStart(range.startContainer, range.startOffset + 1);
    }
    // Move range end until is in the visible area
    while (range.startOffset < range.endOffset)
    {
        var rangeRect = range.getBoundingClientRect();
        if (rangeRect.bottom <= textRect.bottom)
            break;

        range.setEnd(range.endContainer, range.endOffset - 1);
    }
    return range;
}