scrollbar-gutter CSS property allows authors to reserve space for the scrollbar, preventing unwanted layout changes as the content grows while also avoiding unnecessary visuals when scrolling isn't needed.
An element's scrollbar gutter is the space between the inner border edge and the outer padding edge, where the browser may display a scrollbar. If no scrollbar is present, the gutter will be painted as an extension of the padding.
The browser determines whether classic scrollbars or overlay scrollbars are used:
- Classic scrollbars are always placed in a gutter, consuming space when present.
- Overlay scrollbars are placed over the content, not in a gutter, and are usually partially transparent.