Why Sticky Headers Jump When Fonts Load—And How to Stop It

Sticky headers are broken on slow network connections due to asynchronous font loading—a bug that's nearly impossible to reproduce locally but destroys UX in the wild. The culprit is the font-swap behavior that reflows the header after your measurement, creating a mismatch between your spacer and the actual header height. Understanding this timing issue reveals a simple solution: remeasure the header after fonts load using the `document.fonts.ready` API.