Observed Signal · Jun 3, 2026 · Technical Article · Source: DEV Community · Impact: 1/5 · Sentiment: Neutral
One line of code fixes jumping sticky header
A web developer explains a common bug where page content jumps when a header becomes sticky due to asynchronous webfont loading. The measured spacer height is taken before the webfont loads (fallback font), causing a mismatch once the real font swaps in. The author recommends re-measuring the header after fonts finish loading using the Font Loading API (document.fonts.ready) and, if the header is already fixed, temporarily removing the sticky class synchronously to read the element's natural height. The fix was implemented in the Giuliomax Menu Builder WordPress plugin and the author links to a GitHub repo demonstrating the change.
Practical front-end UX/implementation tip useful to web developers and publishers but has limited, narrow impact on the broader AdTech/MarTech industry.
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Key Takeaways & Evidence Grounding
- Asynchronous webfonts (font-display: swap) can change header height after initial render, causing content to jump when a fixed header becomes sticky.
- Measuring header offsetHeight before fonts load yields a provisional spacer height that may be incorrect once the real font loads.
- Solution: call recalcSpacer again when document.fonts.ready resolves to re-measure and update the spacer height.
- If the header may already be fixed when fonts load, temporarily remove the sticky class synchronously, read offsetHeight, then re-add the class to get a correct measurement without visible repaint.
- The fix was shipped in the Giuliomax Menu Builder WordPress plugin; demonstration code and repo links are provided (GitHub and WordPress plugin page).
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