feat: Add feature flag for Dialog scroll performance optimization

.changeset/perf-dialog-has-selector.md

@@ -2,7 +2,10 @@
 '@primer/react': patch
 ---
 
-perf(Dialog): Replace body:has() with direct class and scope footer selector
+perf(Dialog): Add feature flag for CSS :has() selector performance optimization
 
-- Replace `body:has(.Dialog.DisableScroll)` with direct `body.DialogScrollDisabled` class
-- Scope `:has(.Footer)` to direct child with `>` combinator for O(1) lookup
+- Add `primer_react_css_has_selector_perf` feature flag (default: false)
+- When flag is OFF: uses legacy `body:has(.Dialog.DisableScroll)` selector
+- When flag is ON: uses optimized direct `body.DialogScrollDisabled` class with ref counting
+- Scope footer `:has(.Footer)` to direct child with `>` combinator for O(1) lookup
+- Enables gradual rollout and easy rollback of performance optimization

packages/react/src/Dialog/Dialog.module.css

@@ -229,10 +229,26 @@
   }
 }
 
+/* DisableScroll class is added to Dialog when scroll should be disabled on body */
+.DisableScroll {
+  /* This class is used as a selector target for the legacy :has() CSS selector */
+}
+
+/*
+ * LEGACY: Scoped :has() selector with negation guard
+ * Only evaluates when data-dialog-scroll-optimized is NOT present on body.
+ * When the attribute IS present (flag ON), browser skips :has() evaluation
+ * because the :not() check fails first (O(1) attribute lookup).
+ */
+body:not([data-dialog-scroll-optimized]):has(.Dialog.DisableScroll) {
+  /* stylelint-disable-next-line primer/spacing */
+  padding-right: var(--prc-dialog-scrollgutter) !important;
+  overflow: hidden !important;
+}
+
 /*
- * PERFORMANCE: Using a direct class on body instead of body:has(.Dialog.DisableScroll)
- * The :has() selector forces the browser to scan the entire DOM on every style recalc,
- * which is O(n) where n is the number of DOM nodes. This is expensive on large pages.
+ * PERFORMANCE OPTIMIZATION: Direct class on body - O(1) lookup
+ * Active when primer_react_css_has_selector_perf flag is ON
  */
 body:global(.DialogScrollDisabled) {
   /* stylelint-disable-next-line primer/spacing */

packages/react/src/Dialog/Dialog.test.tsx

@@ -6,6 +6,7 @@ import {Dialog} from './Dialog'
 import {Button} from '../Button'
 import {implementsClassName} from '../utils/testing'
 import classes from './Dialog.module.css'
+import {FeatureFlags} from '../FeatureFlags'
 
 describe('Dialog', () => {
   implementsClassName(Dialog, classes.Dialog)
@@ -338,4 +339,107 @@ describe('Footer button loading states', () => {
     expect(publishButton).toHaveAttribute('data-loading', 'true')
     expect(deleteButton).not.toHaveAttribute('data-loading', 'true')
   })
+
+  describe('primer_react_css_has_selector_perf feature flag', () => {
+    it('does not add data-dialog-scroll-optimized attribute when flag is OFF', () => {
+      const {unmount} = render(
+        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
+          <Dialog onClose={() => {}}>Dialog content</Dialog>
+        </FeatureFlags>,
+      )
+
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      unmount()
+
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(false)
+    })
+
+    it('adds data-dialog-scroll-optimized attribute when flag is ON', () => {
+      const {unmount} = render(
+        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
+          <Dialog onClose={() => {}}>Dialog content</Dialog>
+        </FeatureFlags>,
+      )
+
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      unmount()
+
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(false)
+    })
+
+    it('handles multiple dialogs with ref counting when flag is ON', () => {
+      const {unmount: unmount1} = render(
+        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
+          <Dialog onClose={() => {}}>Dialog 1</Dialog>
+        </FeatureFlags>,
+      )
+
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      // Render second dialog
+      const {unmount: unmount2} = render(
+        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
+          <Dialog onClose={() => {}}>Dialog 2</Dialog>
+        </FeatureFlags>,
+      )
+
+      // Attribute should still be present
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      // Unmount first dialog
+      unmount1()
+
+      // Attribute and class should still be present (second dialog is still open)
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      // Unmount second dialog
+      unmount2()
+
+      // Now both should be removed
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(false)
+    })
+
+    it('handles multiple dialogs correctly when flag is OFF', () => {
+      const {unmount: unmount1} = render(
+        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
+          <Dialog onClose={() => {}}>Dialog 1</Dialog>
+        </FeatureFlags>,
+      )
+
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      // Render second dialog
+      const {unmount: unmount2} = render(
+        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
+          <Dialog onClose={() => {}}>Dialog 2</Dialog>
+        </FeatureFlags>,
+      )
+
+      // Attribute should not be present, class should be present
+      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      // Unmount first dialog
+      unmount1()
+
+      // Class should still be present (second dialog is still open)
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(true)
+
+      // Unmount second dialog
+      unmount2()
+
+      // Class should be removed
+      expect(document.body.classList.contains('DialogScrollDisabled')).toBe(false)
+    })
+  })
 })

packages/react/src/Dialog/Dialog.tsx

@@ -16,6 +16,7 @@ import type {ForwardRefComponent as PolymorphicForwardRefComponent} from '../uti
 import classes from './Dialog.module.css'
 import {clsx} from 'clsx'
 import {useSlots} from '../hooks/useSlots'
+import {useFeatureFlag} from '../FeatureFlags'
 
 /* Dialog Version 2 */
 
@@ -289,22 +290,42 @@ const _Dialog = React.forwardRef<HTMLDivElement, React.PropsWithChildren<DialogP
     [onClose],
   )
 
+  const usePerfOptimization = useFeatureFlag('primer_react_css_has_selector_perf')
+
   React.useEffect(() => {
     const scrollbarWidth = window.innerWidth - document.body.clientWidth
-    // If the dialog is rendered, we add a class to the dialog element to disable
-    dialogRef.current?.classList.add(classes.DisableScroll)
-    // and set a CSS variable to the scrollbar width so that the dialog can
-    // account for the scrollbar width when calculating its width.
+    const dialog = dialogRef.current
+
+    // Add DisableScroll class to this dialog
+    dialog?.classList.add(classes.DisableScroll)
     document.body.style.setProperty('--prc-dialog-scrollgutter', `${scrollbarWidth}px`)
 
-    // Add class directly to body for scroll disabling (instead of using CSS :has() selector)
-    // This avoids expensive DOM-wide style recalcs on every interaction
-    document.body.classList.add('DialogScrollDisabled')
+    if (usePerfOptimization) {
+      // Optimized path: set attribute and class on body
+      document.body.setAttribute('data-dialog-scroll-optimized', '')
+      document.body.classList.add('DialogScrollDisabled')
+    } else {
+      // Legacy path: only add class (CSS :has() selector handles the rest)
+      document.body.classList.add('DialogScrollDisabled')
+    }
+
     return () => {
-      document.body.classList.remove('DialogScrollDisabled')
-      document.body.style.removeProperty('--prc-dialog-scrollgutter')
+      // Remove DisableScroll class from this dialog
+      dialog?.classList.remove(classes.DisableScroll)
+
+      // Query DOM to check if any other dialogs with DisableScroll remain
+      const remainingDialogs = document.querySelectorAll(`.${classes.DisableScroll}`)
+
+      if (remainingDialogs.length === 0) {
+        // No more dialogs open, clean up body
+        document.body.style.removeProperty('--prc-dialog-scrollgutter')
+        document.body.classList.remove('DialogScrollDisabled')
+        if (usePerfOptimization) {
+          document.body.removeAttribute('data-dialog-scroll-optimized')
+        }
+      }
     }
-  }, [])
+  }, [usePerfOptimization])
 
   const header = slots.header ?? (renderHeader ?? DefaultHeader)(defaultedProps)
   const body = slots.body ?? (renderBody ?? DefaultBody)({...defaultedProps, children: childrenWithoutSlots})

packages/react/src/FeatureFlags/DefaultFeatureFlags.ts

@@ -3,6 +3,7 @@ import {FeatureFlagScope} from './FeatureFlagScope'
 export const DefaultFeatureFlags = FeatureFlagScope.create({
   primer_react_action_list_item_as_button: false,
   primer_react_breadcrumbs_overflow_menu: false,
+  primer_react_css_has_selector_perf: false,
   primer_react_overlay_overflow: false,
   primer_react_select_panel_fullscreen_on_narrow: false,
   primer_react_select_panel_order_selected_at_top: false,