Create A Fade Out Overflow Effect Using CSS Mask-Image

Learn how to create an overflow fade out effect using CSS mask-image. This helps indicate to the user that there’s more content to be seen. Masking is very useful when overflowing content that’s placed over an image or a non-uniform background.

Let’s take a look at the example below. When scrolling the custom scrollbar we can see that the content area overlows and the content is faded out.

The HTML used for this effect is a normal block-level element that can have any kind of content.

<div class="masked-overflow">
    <p>What is Lorem Ipsum?</p>
</div>

Then using CSS we apply the mask.

.masked-overflow {
    /* scroll bar width, for use in mask calculations */
    --scrollbar-width: 8px;

    /* mask fade distance, for use in mask calculations */
    --mask-height: 32px;

    /* If content exceeds height of container, overflow! */
    overflow-y: auto;

    /* Our height limit */
    height: 300px;

    /* Need to make sure container has bottom space,
  otherwise content at the bottom is always faded out */
    padding-bottom: var(--mask-height);

    /* Keep some space between content and scrollbar */
    padding-right: 20px;

    /* The CSS mask */

    /* The content mask is a linear gradient from top to bottom */
    --mask-image-content: linear-gradient(
        to bottom,
        transparent,
        black var(--mask-height),
        black calc(100% - var(--mask-height)),
        transparent
    );

    /* Here we scale the content gradient to the width of the container 
  minus the scrollbar width. The height is the full container height */
    --mask-size-content: calc(100% - var(--scrollbar-width)) 100%;

    /* The scrollbar mask is a black pixel */
    --mask-image-scrollbar: linear-gradient(black, black);

    /* The width of our black pixel is the width of the scrollbar.
  The height is the full container height */
    --mask-size-scrollbar: var(--scrollbar-width) 100%;

    /* Apply the mask image and mask size variables */
    mask-image: var(--mask-image-content), var(--mask-image-scrollbar);
    mask-size: var(--mask-size-content), var(--mask-size-scrollbar);

    /* Position the content gradient in the top left, and the 
  scroll gradient in the top right */
    mask-position: 0 0, 100% 0;

    /* We don't repeat our mask images */
    mask-repeat: no-repeat, no-repeat;
}

Note that you should either use autoprefixer or prepend the mask properties with -moz- and -webkit- to make them work.

We need these two mask images because when a mask is applied, transparent pixels in the mask are the pixels that become masked. When applying only the content mask the scrollbar gets masked as well because the pixels behind it are transparent.

While the scrollbar still works, visually it’s not that attractive.

Our --mask-image-scrollbar paints a set of black opaque pixels behind the scroll area making the scrollbar always visible.

The custom scrollbar is created with this CSS, we use the current text color as scrollbar color.

/* Firefox */
.masked-overflow {
    scrollbar-width: thin; /* can also be normal, or none, to not render scrollbar */
    scrollbar-color: currentColor transparent; /* foreground background */
}

/* Webkit / Blink */
.masked-overflow::-webkit-scrollbar {
    width: var(--scrollbar-width);
}

.masked-overflow::-webkit-scrollbar-thumb {
    background-color: currentColor;
    border-radius: 9999px; /* always round */
}

.masked-overflow::-webkit-scrollbar-track {
    background-color: transparent;
}