diff --git a/.changeset/graduate-css-has-selector-perf-flag.md b/.changeset/graduate-css-has-selector-perf-flag.md
new file mode 100644
index 00000000000..0aa702353a7
--- /dev/null
+++ b/.changeset/graduate-css-has-selector-perf-flag.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Graduate `primer_react_css_has_selector_perf` feature flag: the CSS `:has()` performance optimization (`body[data-dialog-scroll-disabled]`) is now the default behavior for Dialog scroll disabling
diff --git a/packages/react/src/Dialog/Dialog.module.css b/packages/react/src/Dialog/Dialog.module.css
index ac5a664c148..d46a02d81f0 100644
--- a/packages/react/src/Dialog/Dialog.module.css
+++ b/packages/react/src/Dialog/Dialog.module.css
@@ -285,27 +285,8 @@
   }
 }
 
-/* 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).
- */
-/* stylelint-disable-next-line selector-no-qualifying-type */
-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 OPTIMIZATION: Direct attribute on body - O(1) lookup
- * Active when primer_react_css_has_selector_perf flag is ON
  */
 /* stylelint-disable-next-line selector-no-qualifying-type */
 body[data-dialog-scroll-disabled] {
diff --git a/packages/react/src/Dialog/Dialog.test.tsx b/packages/react/src/Dialog/Dialog.test.tsx
index 65297c762c7..b7dc38d5f7e 100644
--- a/packages/react/src/Dialog/Dialog.test.tsx
+++ b/packages/react/src/Dialog/Dialog.test.tsx
@@ -1,19 +1,13 @@
 import React from 'react'
 import {render, fireEvent, waitFor} from '@testing-library/react'
-import {describe, expect, it, vi, beforeEach} from 'vitest'
+import {describe, expect, it, vi} from 'vitest'
 import userEvent from '@testing-library/user-event'
 import {Dialog} from './Dialog'
 import {Button} from '../Button'
 import {implementsClassName} from '../utils/testing'
 import classes from './Dialog.module.css'
-import {FeatureFlags} from '../FeatureFlags'
-import {__resetDialogScrollOptimizedCount} from '../FeatureFlags/FeatureFlags'
 
 describe('Dialog', () => {
-  beforeEach(() => {
-    __resetDialogScrollOptimizedCount()
-  })
-
   implementsClassName(Dialog, classes.Dialog)
   it('renders with role "dialog" by default', () => {
     const {getByRole} = render(<Dialog onClose={() => {}}>Pay attention to me</Dialog>)
@@ -376,126 +370,32 @@ describe('Footer button loading states', () => {
     expect(deleteButton).not.toHaveAttribute('data-loading', 'true')
   })
 
-  describe('primer_react_css_has_selector_perf feature flag', () => {
-    it('does not add data-dialog-scroll-optimized or data-dialog-scroll-disabled 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.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
+  describe('scroll disable behavior', () => {
+    it('sets data-dialog-scroll-disabled on body when dialog mounts', () => {
+      const {unmount} = render(<Dialog onClose={() => {}}>Dialog content</Dialog>)
 
-      unmount()
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-      expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
-    })
-
-    it('adds data-dialog-scroll-optimized at provider level and data-dialog-scroll-disabled when dialog mounts', () => {
-      const {unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <Dialog onClose={() => {}}>Dialog content</Dialog>
-        </FeatureFlags>,
-      )
-
-      // Provider sets data-dialog-scroll-optimized, Dialog sets data-dialog-scroll-disabled
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
       expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(true)
 
       unmount()
 
-      // Both should be removed on unmount
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-      expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
-    })
-
-    it('sets data-dialog-scroll-optimized even when no dialogs are open', () => {
-      const {unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>No dialogs here</div>
-        </FeatureFlags>,
-      )
-
-      // Provider sets the attribute even without dialogs
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
       expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
-
-      unmount()
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).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>,
-      )
+    it('handles multiple dialogs with ref counting', () => {
+      const {unmount: unmount1} = render(<Dialog onClose={() => {}}>Dialog 1</Dialog>)
 
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
       expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(true)
 
-      // Render second dialog
-      const {unmount: unmount2} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <Dialog onClose={() => {}}>Dialog 2</Dialog>
-        </FeatureFlags>,
-      )
+      const {unmount: unmount2} = render(<Dialog onClose={() => {}}>Dialog 2</Dialog>)
 
-      // Attributes should still be present
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
       expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(true)
 
-      // Unmount first dialog
+      // Unmount first dialog - attribute should still be present
       unmount1()
-
-      // Attributes should still be present (second dialog and provider are still mounted)
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
       expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(true)
 
-      // Unmount second dialog
-      unmount2()
-
-      // Now both should be removed
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-      expect(document.body.hasAttribute('data-dialog-scroll-disabled')).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.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
-
-      // Render second dialog
-      const {unmount: unmount2} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
-          <Dialog onClose={() => {}}>Dialog 2</Dialog>
-        </FeatureFlags>,
-      )
-
-      // Attributes should not be present
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-      expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
-
-      // Unmount first dialog
-      unmount1()
-
-      // Attributes should still not be present
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-      expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
-
-      // Unmount second dialog
+      // Unmount second dialog - attribute should be removed
       unmount2()
-
-      // Attributes should still not be present
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
       expect(document.body.hasAttribute('data-dialog-scroll-disabled')).toBe(false)
     })
   })
diff --git a/packages/react/src/Dialog/Dialog.tsx b/packages/react/src/Dialog/Dialog.tsx
index 2a070a88d66..77791ebae11 100644
--- a/packages/react/src/Dialog/Dialog.tsx
+++ b/packages/react/src/Dialog/Dialog.tsx
@@ -19,6 +19,13 @@ import {useSlots} from '../hooks/useSlots'
 
 /* Dialog Version 2 */
 
+/**
+ * Ref count for data-dialog-scroll-disabled attribute management.
+ * Tracks how many dialogs are currently open to know when to remove the attribute.
+ * This is client-only: it is only accessed inside useEffect, which never runs on the server.
+ */
+let dialogScrollDisabledCount = 0
+
 /**
  * Props that characterize a button to be rendered into the footer of
  * a Dialog.
@@ -267,7 +274,6 @@ const _Dialog = React.forwardRef<HTMLDivElement, React.PropsWithChildren<DialogP
   const autoFocusedFooterButtonRef = useRef<HTMLButtonElement>(null)
   for (const footerButton of footerButtons) {
     if (footerButton.autoFocus) {
-      // eslint-disable-next-line react-hooks/immutability
       footerButton.ref = autoFocusedFooterButtonRef
     }
   }
@@ -308,29 +314,16 @@ const _Dialog = React.forwardRef<HTMLDivElement, React.PropsWithChildren<DialogP
 
   React.useEffect(() => {
     const scrollbarWidth = window.innerWidth - document.body.clientWidth
-    const dialog = dialogRef.current
-    const usePerfOptimization = document.body.hasAttribute('data-dialog-scroll-optimized')
 
-    // Add DisableScroll class to this dialog (for legacy :has() selector path)
-    dialog?.classList.add(classes.DisableScroll)
+    dialogScrollDisabledCount++
     document.body.style.setProperty('--prc-dialog-scrollgutter', `${scrollbarWidth}px`)
-
-    if (usePerfOptimization) {
-      // Optimized path: set attribute on body for direct CSS targeting
-      document.body.setAttribute('data-dialog-scroll-disabled', '')
-    }
-    // Legacy path: no action needed - CSS :has(.Dialog.DisableScroll) handles it
+    document.body.setAttribute('data-dialog-scroll-disabled', '')
 
     return () => {
-      dialog?.classList.remove(classes.DisableScroll)
-
-      const remainingDialogs = document.querySelectorAll(`.${classes.DisableScroll}`)
-
-      if (remainingDialogs.length === 0) {
+      dialogScrollDisabledCount--
+      if (dialogScrollDisabledCount === 0) {
         document.body.style.removeProperty('--prc-dialog-scrollgutter')
-        if (usePerfOptimization) {
-          document.body.removeAttribute('data-dialog-scroll-disabled')
-        }
+        document.body.removeAttribute('data-dialog-scroll-disabled')
       }
     }
   }, [])
diff --git a/packages/react/src/FeatureFlags/DefaultFeatureFlags.ts b/packages/react/src/FeatureFlags/DefaultFeatureFlags.ts
index 796448887e8..95bf167f84a 100644
--- a/packages/react/src/FeatureFlags/DefaultFeatureFlags.ts
+++ b/packages/react/src/FeatureFlags/DefaultFeatureFlags.ts
@@ -2,7 +2,6 @@ import {FeatureFlagScope} from './FeatureFlagScope'
 
 export const DefaultFeatureFlags = FeatureFlagScope.create({
   primer_react_breadcrumbs_overflow_menu: false,
-  primer_react_css_has_selector_perf: false,
   primer_react_select_panel_fullscreen_on_narrow: false,
   primer_react_select_panel_order_selected_at_top: false,
   primer_react_select_panel_remove_active_descendant: false,
diff --git a/packages/react/src/FeatureFlags/FeatureFlags.tsx b/packages/react/src/FeatureFlags/FeatureFlags.tsx
index c0e2ac4798f..a3f34efeb7d 100644
--- a/packages/react/src/FeatureFlags/FeatureFlags.tsx
+++ b/packages/react/src/FeatureFlags/FeatureFlags.tsx
@@ -2,34 +2,11 @@ import type React from 'react'
 import {useContext, useMemo} from 'react'
 import {FeatureFlagContext} from './FeatureFlagContext'
 import {FeatureFlagScope, type FeatureFlags} from './FeatureFlagScope'
-import useIsomorphicLayoutEffect from '../utils/useIsomorphicLayoutEffect'
 
 export type FeatureFlagsProps = React.PropsWithChildren<{
   flags: FeatureFlags
 }>
 
-/**
- * Ref count for data-dialog-scroll-optimized attribute management.
- *
- * NOTE: This is temporary infrastructure while we feature flag the CSS :has()
- * performance optimization (primer_react_css_has_selector_perf). Once the flag
- * is removed and the optimization is the default behavior, this ref counting
- * can be removed - the attribute can simply always be present.
- *
- * @internal - Not part of the public API
- */
-let dialogScrollOptimizedCount = 0
-
-/**
- * Reset the ref count for testing purposes only.
- *
- * @internal - Not part of the public API. Only exported for test isolation.
- */
-export function __resetDialogScrollOptimizedCount(): void {
-  dialogScrollOptimizedCount = 0
-  document.body.removeAttribute('data-dialog-scroll-optimized')
-}
-
 export function FeatureFlags({children, flags}: FeatureFlagsProps) {
   const parentFeatureFlags = useContext(FeatureFlagContext)
   const value = useMemo(() => {
@@ -37,22 +14,5 @@ export function FeatureFlags({children, flags}: FeatureFlagsProps) {
     return scope
   }, [parentFeatureFlags, flags])
 
-  const isOptimizationEnabled = value.enabled('primer_react_css_has_selector_perf')
-
-  // Set body attribute for CSS :has() optimization when flag is enabled
-  useIsomorphicLayoutEffect(() => {
-    if (isOptimizationEnabled) {
-      dialogScrollOptimizedCount++
-      document.body.setAttribute('data-dialog-scroll-optimized', '')
-
-      return () => {
-        dialogScrollOptimizedCount--
-        if (dialogScrollOptimizedCount === 0) {
-          document.body.removeAttribute('data-dialog-scroll-optimized')
-        }
-      }
-    }
-  }, [isOptimizationEnabled])
-
   return <FeatureFlagContext.Provider value={value}>{children}</FeatureFlagContext.Provider>
 }
diff --git a/packages/react/src/FeatureFlags/__tests__/FeatureFlags.test.tsx b/packages/react/src/FeatureFlags/__tests__/FeatureFlags.test.tsx
index 035b43bf0eb..cc7b69e802f 100644
--- a/packages/react/src/FeatureFlags/__tests__/FeatureFlags.test.tsx
+++ b/packages/react/src/FeatureFlags/__tests__/FeatureFlags.test.tsx
@@ -1,14 +1,8 @@
-import {describe, expect, it, beforeEach} from 'vitest'
+import {describe, expect, it} from 'vitest'
 import {render} from '@testing-library/react'
 import {FeatureFlags, useFeatureFlag} from '../../FeatureFlags'
-import {__resetDialogScrollOptimizedCount} from '../FeatureFlags'
 
 describe('FeatureFlags', () => {
-  beforeEach(() => {
-    // Reset module state between tests for isolation
-    __resetDialogScrollOptimizedCount()
-  })
-
   it('should allow a component to check if a feature flag is enabled', () => {
     const calls: Array<boolean> = []
 
@@ -48,188 +42,4 @@ describe('FeatureFlags', () => {
 
     expect(calls).toEqual([false])
   })
-
-  describe('data-dialog-scroll-optimized attribute management', () => {
-    it('should set data-dialog-scroll-optimized attribute when primer_react_css_has_selector_perf is enabled', () => {
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      const {unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Content</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      unmount()
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should not set data-dialog-scroll-optimized attribute when primer_react_css_has_selector_perf is disabled', () => {
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      const {unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
-          <div>Content</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      unmount()
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should handle ref counting correctly with multiple FeatureFlags providers', () => {
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      // Mount first provider
-      const {unmount: unmount1} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Provider 1</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      // Mount second provider
-      const {unmount: unmount2} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Provider 2</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      // Unmount first provider - attribute should still be present
-      unmount1()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      // Unmount second provider - attribute should be removed
-      unmount2()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should handle nested providers with different flag values correctly', () => {
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      const {unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Outer provider with flag enabled</div>
-          <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
-            <div>Inner provider with flag disabled</div>
-          </FeatureFlags>
-        </FeatureFlags>,
-      )
-
-      // Outer provider sets the attribute, inner provider inherits but doesn't override
-      // (inner provider flag is false, so it won't add to count)
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      unmount()
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should handle nested providers where parent has flag disabled and child has flag enabled', () => {
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      const {unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
-          <div>Outer provider with flag disabled</div>
-          <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-            <div>Inner provider with flag enabled</div>
-          </FeatureFlags>
-        </FeatureFlags>,
-      )
-
-      // Inner provider enables the flag, so attribute should be set
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      unmount()
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should only remove attribute when all providers with flag enabled have unmounted', () => {
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      // Mount three providers with flag enabled
-      const {unmount: unmount1} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Provider 1</div>
-        </FeatureFlags>,
-      )
-
-      const {unmount: unmount2} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Provider 2</div>
-        </FeatureFlags>,
-      )
-
-      const {unmount: unmount3} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Provider 3</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      // Unmount first provider
-      unmount1()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      // Unmount second provider
-      unmount2()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      // Unmount third provider - now attribute should be removed
-      unmount3()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should handle flag value changing from false to true', () => {
-      const {rerender, unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
-          <div>Content</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      rerender(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Content</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      unmount()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-
-    it('should handle flag value changing from true to false', () => {
-      const {rerender, unmount} = render(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: true}}>
-          <div>Content</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(true)
-
-      rerender(
-        <FeatureFlags flags={{primer_react_css_has_selector_perf: false}}>
-          <div>Content</div>
-        </FeatureFlags>,
-      )
-
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-
-      unmount()
-      expect(document.body.hasAttribute('data-dialog-scroll-optimized')).toBe(false)
-    })
-  })
 })