What is the function of "overlay" value of "overflow" property?

Question

I am unable to understand the difference between "overlay" & "auto". Does "overlay" does the same work as "auto"?

Answer 1

The only difference is that overflow: overlay is only supported by -Webkit browsers, is non-standardized, and allows the content to extend beneath the scrollbar - whereas overflow: auto will not allow the content to extend beneath the scrollbar, if it appears it'll occupy the space required and shift the content accordingly (either vertically or horizontally).

p {
    display: inline-block;
    width: 12em;
    height: 5em;
    border: dotted;
}

p.overflow-auto { overflow: auto; /* append scrollbars if necessary and shift content accordingly to accommodate */ }

p.overflow-overlay { overflow: overlay; /* append scrollbars if necessary and overlay over/above content */ }

<p class="overflow-auto">overflow: auto
Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium.</p>

<p class="overflow-overlay">overflow: overlay
Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium.</p>

The above snippet demonstrates the difference as follows:

p.overflow-auto { overflow: auto; /* append scrollbars if necessary and shift content accordingly to accommodate */ }

p.overflow-overlay { overflow: overlay; /* append scrollbars if necessary and overlay over/above content */ }