Article illustration 1

Main article image showing scrollbar gutter behavior (credit: Zach Leatherman)

For decades, web developers have wrestled with an insidious layout problem: the jarring content shift that occurs when scrollbars appear or disappear. Whether triggered by modal dialogs, progressive loading, or dynamic content changes, this sudden reflow creates a disorienting user experience and negatively impacts Core Web Vitals metrics like Cumulative Layout Shift (CLS).

As Zach Leatherman notes, the issue becomes especially apparent for users with OS settings that hide scrollbars until needed—the default behavior for most systems. Common symptoms include horizontal page jumps when toggling modals (as sites apply overflow: hidden), content shifting during slow page loads, and unexpected horizontal overflow scrollbars.

The traditional fix—html { overflow-y: scroll; } in CSS resets—forces a permanent scrollbar gutter but comes with significant drawbacks:

/* Classic solution with downside */
html {
  overflow-y: scroll; /* Always shows scrollbar */
}

This guarantees layout stability but wastes space with an ever-present scrollbar, even on short pages. Enter the modern solution: the scrollbar-gutter property. Leatherman proposes an elegant progressive enhancement approach:

/* Modern progressive enhancement */
html {
  overflow-y: scroll; /* Fallback for older browsers */
}

@supports (scrollbar-gutter: stable) {
  html {
    overflow-y: auto; /* Dynamic scrollbar */
    scrollbar-gutter: stable; /* Reserve space */
  }
}

Here's why this technique shines:
1. Backward Compatibility: Older browsers get the reliable overflow-y: scroll fallback
2. Modern Efficiency: Browsers supporting scrollbar-gutter (now Baseline 2023+) reserve space without showing scrollbars until needed
3. Layout Stability: The reserved "gutter" prevents content shifts when scrollbars appear
4. Clean Aesthetics: No unnecessary scrollbars on short content

This solution directly addresses CLS concerns while respecting user preferences and device constraints. As Leatherman emphasizes, it's particularly valuable for:
- Modal/dialog interactions
- Dynamic content injection
- Pages with varying content heights
- Progressive rendering scenarios

Browser support is now robust across Chromium, Firefox, and Safari. For teams monitoring Web Vitals, implementing this snippet could yield measurable CLS improvements with minimal effort. As scrollbar behavior continues evolving—including discussions around drawing over gutters and dual-edge positioning—this approach offers a future-proof foundation for stable, accessible interfaces.

While seemingly minor, eliminating these micro-jitters represents the hallmark of polished UX. As Leatherman's solution demonstrates, sometimes the most impactful fixes come in small CSS packages—quietly ensuring content stays precisely where users expect it.