Is there any way to place markers on top of scrollbars?

Question

Similar to features found in many IDEs and text editors (such as https://github.com/surdu/scroll-marker), I want to be able to show small markers on the scrollbar (of a overflow: scroll element).

I think this could be done by putting a pointer-events: none element as an overlay on the scrollable element and putting absolute-positioned elements on it, but it would be tricky getting the position right considering the scrollbar has platform/browser-specific width, padding, nub size, and things like that. Besides, it feels like a rather hacky solution.

Any pointers?

Answer 1

Here you go. It was a great challenge. It's not perfect, but I hope it shows you a way forward.
So, first a brief explanation.

There is a container with fixed dimensions. There are 2 elements inside it: .inner-container for the text and the .scroll-marker that will have the marks over the scrollbar.

The text are <p> tags, and the highlighted text are <span> tags with .red, .yellow or blue classes. These classes are being used to know the color for each mark. Each mark is created in a loop and appended on .scroll-marker, they are absolutely positioned. A rule calculation of three is done to make the scroll positioning happen correctly.

var container = document.querySelector('.container');
var containerInner = document.querySelector('.container-inner');

var containerHeight = container.offsetHeight;
var containerScrollHeight = containerInner.scrollHeight;

var scrollMarker = document.querySelector('.scroll-marker');

var colorfulStuff = document.querySelectorAll('.container-inner span'); // colorful spans from text

colorfulStuff.forEach(function (span) { // loop to create each marker

    var spanTop = span.offsetTop;
    var spanBottom = spanTop + span.offsetHeight;

    var markerTop = Math.ceil(spanTop * containerHeight / containerScrollHeight);
    var markerBottom = Math.ceil(spanBottom * containerHeight / containerScrollHeight);

    if (span.className === "red") { // choose the correct color
        var markerColor = '#f65e5a';
    } else if (span.className === "yellow") {
        var markerColor = '#fec740';
    } else if (span.className === "blue") {
        var markerColor = '#2985d0';
    }

    var markerElement = document.createElement("span"); // create the marker, set color and position and put it there
    markerElement.style.backgroundColor = markerColor;
    markerElement.style.top = markerTop + "px"
    markerElement.style.height = (markerBottom - markerTop) + "px"
    scrollMarker.appendChild(markerElement);

})

.container {
    background: #2d3744;
    border-radius: 4px;
    position: relative;
    margin: 30px auto;
    height: 320px;
    width: 300px;
    box-shadow: 2px 2px 5px 0 rgba(0, 0, 0, 0.65);
}

.container-inner {
    padding: 20px 30px;
    height: calc(100% - 40px);
    overflow-y: auto;
    overflow-x: hidden;
}

p {
    color: white;
    margin: 12px 0;
    font-size: 18px;
    font-family: sans-serif;
    letter-spacing: 0.44px;
}

.red {
    color: #f65e5a;
}

.yellow {
    color: #fec740;
}

.blue {
    color: #2985d0;
}

.scroll-marker {
    position: absolute;
    height: 100%;
    width: 12px;
    pointer-events: none;
    top: 0;
    right: 0;
}

.scroll-marker span {
    /* markers */
    position: absolute;
    width: 100%;
}

/* scrollbar style */

.container-inner::-webkit-scrollbar-track {
    -webkit-box-shadow: inset 0 0 6px rgba(0, 0, 0, 0.3);
    background-color: #2d3744;
}

.container-inner::-webkit-scrollbar {
    width: 12px;
    background-color: #2d3744;
}

.container-inner::-webkit-scrollbar-thumb {
    border-radius: 10px;
    -webkit-box-shadow: inset 0 0 6px rgba(0, 0, 0, .3);
    background-color: #4c5f74;
}

<div class="container">
  <div class="container-inner">
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores <span class="yellow">provident magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus</span> voluptate non magni. Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident <span class="blue">magnam impedit deleniti modi</span>, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni. Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. <span class="yellow">Inventore asperiores provident</span> magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni. Voluptatem, itaque.</p>
    <p><span class="red">Lorem ipsum</span> dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa <span class="red">perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni.</span> Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni. Voluptatem, itaque.</p>
    <p><span class="yellow">Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus</span> voluptate non magni. Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni. <span class="blue">Voluptatem, itaque.</span></p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni. Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa perspiciatis ex illum iusto vel nulla expedita itaque temporibus voluptate non magni. Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores <span class="yellow">provident magnam</span> impedit deleniti modi, culpa <span class="blue">perspiciatis ex illum iusto vel nulla expedita itaque</span> temporibus voluptate non magni. Voluptatem, itaque.</p>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Inventore asperiores provident magnam impedit deleniti modi, culpa perspiciatis <span class="red">ex illum</span> iusto vel nulla expedita itaque temporibus voluptate non magni. Voluptatem, itaque.</p>
  </div>
  <div class="scroll-marker"></div>
</div>