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PageLayout: move pane resize viewport clamping to CSS #7848

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ef3aa9a
Initial plan
Copilot May 18, 2026
e7da269
Implement CSS-first PageLayout pane resize clamping
Copilot May 18, 2026
0623106
Polish CSS-first PageLayout resize tests
Copilot May 18, 2026
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5 changes: 5 additions & 0 deletions .changeset/swift-forks-sneeze.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'@primer/react': patch
---

PageLayout: Move pane and sidebar viewport clamping to CSS so window resize stays smooth and JS sync runs only after resizing ends
37 changes: 31 additions & 6 deletions packages/react/src/PageLayout/PageLayout.module.css
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Expand Up @@ -31,10 +31,23 @@
--pane-width-small: 100%;
--pane-width-medium: 100%;
--pane-width-large: 100%;
--page-layout-viewport-width: 100vw;
/* NOTE: This value is exported via :export for use in usePaneWidth.ts */
--pane-max-width-diff: 511px;
/* Sidebar uses a smaller diff since it doesn't need to reserve space for a file tree pane */
--sidebar-max-width-diff: 256px;
--pane-viewport-max: max(
var(--pane-min-width, 0px),
calc(var(--page-layout-viewport-width) - var(--pane-max-width-diff))
);
--sidebar-viewport-max: max(
var(--pane-min-width, 0px),
calc(var(--page-layout-viewport-width) - var(--sidebar-max-width-diff))
);

@supports (width: 100dvw) {
--page-layout-viewport-width: 100dvw;
}

@media screen and (min-width: 768px) {
--pane-width-small: 240px;
Expand Down Expand Up @@ -602,11 +615,13 @@
width: 100%;

@media screen and (min-width: 768px) {
/*
* --pane-max-width is set by JS on mount and updated on resize (debounced).
* JS calculates viewport - margin to avoid scrollbar discrepancy with 100vw.
*/
width: clamp(var(--pane-min-width), var(--pane-width), var(--pane-max-width));
width: clamp(var(--pane-min-width), var(--pane-width), min(var(--pane-max-width), var(--pane-viewport-max)));
}

&:where([data-constrain-to-viewport='false']) {
@media screen and (min-width: 768px) {
width: clamp(var(--pane-min-width), var(--pane-width), var(--pane-max-width));
}
}
}
}
Expand Down Expand Up @@ -846,7 +861,17 @@
width: 100%;

@media screen and (min-width: 768px) {
width: clamp(var(--pane-min-width), var(--pane-width), var(--pane-max-width));
width: clamp(
var(--pane-min-width),
var(--pane-width),
min(var(--pane-max-width), var(--sidebar-viewport-max))
);
}

&:where([data-constrain-to-viewport='false']) {
@media screen and (min-width: 768px) {
width: clamp(var(--pane-min-width), var(--pane-width), var(--pane-max-width));
}
}
}

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8 changes: 4 additions & 4 deletions packages/react/src/PageLayout/PageLayout.tsx
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Expand Up @@ -897,11 +897,12 @@ const Pane = React.forwardRef<HTMLDivElement, React.PropsWithChildren<PageLayout
{...(id && {id: paneId})}
className={classes.Pane}
data-resizable={resizable || undefined}
data-constrain-to-viewport={isCustomWidthOptions(width) ? 'false' : undefined}
style={
{
'--spacing': `var(--spacing-${padding})`,
'--pane-min-width': isCustomWidthOptions(width) ? width.min : `${minWidth}px`,
'--pane-max-width': isCustomWidthOptions(width) ? width.max : `calc(100vw - var(--pane-max-width-diff))`,
'--pane-max-width': isCustomWidthOptions(width) ? width.max : 'var(--page-layout-viewport-width)',
'--pane-width-custom': isCustomWidthOptions(width) ? width.default : undefined,
'--pane-width-size': `var(--pane-width-${isPaneWidth(width) ? width : 'custom'})`,
'--pane-width': `${currentWidth}px`,
Expand Down Expand Up @@ -1201,13 +1202,12 @@ const Sidebar = React.forwardRef<HTMLDivElement, React.PropsWithChildren<PageLay
{...(id && {id: sidebarId})}
className={classes.Sidebar}
data-resizable={resizable || undefined}
data-constrain-to-viewport={isCustomWidthOptions(width) ? 'true' : undefined}
style={
{
'--spacing': `var(--spacing-${padding})`,
'--pane-min-width': isCustomWidthOptions(width) ? width.min : `${minWidth}px`,
'--pane-max-width': isCustomWidthOptions(width)
? width.max
: `calc(100vw - var(--sidebar-max-width-diff))`,
'--pane-max-width': isCustomWidthOptions(width) ? width.max : 'var(--page-layout-viewport-width)',
'--pane-width-custom': isCustomWidthOptions(width) ? width.default : undefined,
'--pane-width-size': `var(--pane-width-${isPaneWidth(width) ? width : 'custom'})`,
'--pane-width': `${currentWidth}px`,
Expand Down
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