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perf(Breadcrumbs): Optimize CSS :has() selector and batch offsetWidth reads #7326

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perf(Breadcrumbs): Optimize CSS :has() selector and batch offsetWidth reads #7326

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perf(Breadcrumbs): Optimize CSS :has() selector and batch offsetWidth…
mattcosta7 Dec 15, 2025
ee72d57
Merge branch 'main' into perf/breadcrumbs-cls-fix
mattcosta7 Dec 19, 2025
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8 changes: 8 additions & 0 deletions .changeset/perf-breadcrumbs-cls-fix.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
---
'@primer/react': patch
---

perf(Breadcrumbs): Optimize CSS :has() selector and batch offsetWidth reads

- Scope `:has(.MenuOverlay)` to shallow path `> .MenuDetails .MenuOverlay` for O(1) lookup
- Batch all offsetWidth reads in a single pass to avoid layout thrashing
3 changes: 2 additions & 1 deletion packages/react/src/Breadcrumbs/Breadcrumbs.module.css
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Expand Up @@ -117,7 +117,8 @@
list-style: none;

/* allow menu items to wrap line */
&:has(.MenuOverlay) {
/* PERFORMANCE: MenuOverlay is inside MenuDetails > summary sibling, use shallow path */
&:has(> .MenuDetails .MenuOverlay) {
white-space: normal;
}

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4 changes: 3 additions & 1 deletion packages/react/src/Breadcrumbs/Breadcrumbs.tsx
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Expand Up @@ -188,9 +188,11 @@ function Breadcrumbs({className, children, style, overflow = 'wrap', variant = '
listElement.children.length === childArray.length
) {
const listElementArray = Array.from(listElement.children) as HTMLElement[]
// Batch all offsetWidth reads in a single pass to avoid layout thrashing
const widths = listElementArray.map(child => child.offsetWidth)
setChildArrayWidths(widths)
setRootItemWidth(listElementArray[0].offsetWidth)
// Use first width from the array instead of reading offsetWidth again
setRootItemWidth(widths[0])
}
}, [childArray, overflowMenuEnabled])

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