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graduate primer_react_theme_provider_remove_ssr_handoff ff #8057

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0cf2fbc
graduate primer_react_theme_provider_remove_ssr_handoff ff
francinelucca Jun 25, 2026
73283f7
add changeset
francinelucca Jun 25, 2026
fda08a3
Update useTheme hook docs for deprecated preventSSRMismatch
Copilot Jun 25, 2026
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5 changes: 5 additions & 0 deletions .changeset/graduate-theming-feature-flags.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'@primer/react': minor
---

ThemeProvider: Remove `primer_react_theme_provider_remove_ssr_handoff` feature flag.
1 change: 0 additions & 1 deletion packages/react/src/FeatureFlags/DefaultFeatureFlags.ts
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Expand Up @@ -8,5 +8,4 @@ export const DefaultFeatureFlags = FeatureFlagScope.create({
primer_react_action_list_group_heading_trailing_action: false,
primer_react_action_list_item_gap: false,
primer_react_timeline_list_semantics: false,
primer_react_theme_provider_remove_ssr_handoff: false,
})
61 changes: 3 additions & 58 deletions packages/react/src/ThemeProvider.tsx
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Original file line number Diff line number Diff line change
@@ -1,8 +1,6 @@
import React from 'react'
import defaultTheme from './theme'
import deepmerge from 'deepmerge'
import {useId} from './hooks'
import {useFeatureFlag} from './FeatureFlags'
import {useSyncedState} from './hooks/useSyncedState'
import {ThemeContext} from './ThemeContext'
import {useTheme} from './useTheme'
Expand All @@ -21,7 +19,7 @@ export type ThemeProviderProps = {
dayScheme?: string
nightScheme?: string
/**
* No-op when the `primer_react_theme_provider_remove_ssr_handoff` feature flag is enabled.
* @deprecated This prop is no longer used and has no effect.
*/
preventSSRMismatch?: boolean
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/**
Expand All @@ -32,33 +30,6 @@ export type ThemeProviderProps = {
contextOnly?: boolean
}

// inspired from __NEXT_DATA__, we use application/json to avoid CSRF policy with inline scripts
const serverHandoffCache = new Map<string, Record<string, unknown>>()
const emptyHandoff: Record<string, unknown> = {}
const getServerHandoff = (id: string) => {
if (typeof document === 'undefined') return emptyHandoff

const cached = serverHandoffCache.get(id)
if (cached !== undefined) return cached

try {
const serverData = document.getElementById(`__PRIMER_DATA_${id}__`)?.textContent
if (serverData) {
const parsed = JSON.parse(serverData)
serverHandoffCache.set(id, parsed)
return parsed
}
} catch (_error) {
// if document/element does not exist or JSON is invalid, suppress error
}

const empty = {}
serverHandoffCache.set(id, empty)
return empty
}

const emptySubscribe = () => () => {}

export const ThemeProvider: React.FC<React.PropsWithChildren<ThemeProviderProps>> = ({children, ...props}) => {
// Get fallback values from parent ThemeProvider (if exists)
const {
Expand All @@ -71,22 +42,11 @@ export const ThemeProvider: React.FC<React.PropsWithChildren<ThemeProviderProps>
// Initialize state
const theme = fallbackTheme ?? defaultTheme

const removeSSRHandoff = useFeatureFlag('primer_react_theme_provider_remove_ssr_handoff')
const uniqueDataId = useId()

const [colorMode, setColorMode] = useSyncedState(props.colorMode ?? fallbackColorMode ?? defaultColorMode)
const [dayScheme, setDayScheme] = useSyncedState(props.dayScheme ?? fallbackDayScheme ?? defaultDayScheme)
const [nightScheme, setNightScheme] = useSyncedState(props.nightScheme ?? fallbackNightScheme ?? defaultNightScheme)
const systemColorMode = useSystemColorMode()
const clientColorMode = resolveColorMode(colorMode, systemColorMode)
// During SSR/hydration, use the server-rendered color mode from the handoff script tag
// to avoid mismatches. After hydration, resolve from client state.
const ssrResolvedColorMode = React.useSyncExternalStore(
emptySubscribe,
() => clientColorMode,
() => getServerHandoff(uniqueDataId).resolvedServerColorMode ?? clientColorMode,
)
const resolvedColorMode = removeSSRHandoff ? clientColorMode : ssrResolvedColorMode
const resolvedColorMode = resolveColorMode(colorMode, systemColorMode)
const colorScheme = chooseColorScheme(resolvedColorMode, dayScheme, nightScheme)
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const {resolvedTheme, resolvedColorScheme} = React.useMemo(
() => applyColorScheme(theme, colorScheme),
Expand Down Expand Up @@ -120,22 +80,8 @@ export const ThemeProvider: React.FC<React.PropsWithChildren<ThemeProviderProps>
],
)

const ssrHandoffScript =
!removeSSRHandoff && props.preventSSRMismatch ? (
<script
type="application/json"
id={`__PRIMER_DATA_${uniqueDataId}__`}
dangerouslySetInnerHTML={{__html: JSON.stringify({resolvedServerColorMode: resolvedColorMode})}}
/>
) : null

if (props.contextOnly) {
return (
<ThemeContext.Provider value={contextValue}>
{children}
{ssrHandoffScript}
</ThemeContext.Provider>
)
return <ThemeContext.Provider value={contextValue}>{children}</ThemeContext.Provider>
}

return (
Expand All @@ -146,7 +92,6 @@ export const ThemeProvider: React.FC<React.PropsWithChildren<ThemeProviderProps>
data-dark-theme={nightScheme}
>
{children}
{ssrHandoffScript}
</div>
</ThemeContext.Provider>
)
Expand Down
58 changes: 0 additions & 58 deletions packages/react/src/__tests__/ThemeProvider.test.tsx
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Expand Up @@ -3,7 +3,6 @@ import userEvent from '@testing-library/user-event'
import {describe, expect, it, vi} from 'vitest'
import React from 'react'
import ThemeProvider from '../ThemeProvider'
import {FeatureFlags} from '../FeatureFlags'
import {useColorSchemeVar, useTheme} from '../useTheme'

// copied from '@primer/primitives/dist/css/functional/themes/';
Expand Down Expand Up @@ -571,61 +570,4 @@ describe('contextOnly', () => {

expect(screen.getByTestId('consumer').textContent).toBe('night-light-dark_dimmed')
})

it('renders the preventSSRMismatch script tag when contextOnly and preventSSRMismatch are both true', () => {
const {container} = render(
<ThemeProvider contextOnly preventSSRMismatch>
<span>Hello</span>
</ThemeProvider>,
)

const div = container.querySelector('[data-color-mode]')
expect(div).not.toBeInTheDocument()

const script = container.querySelector('script[type="application/json"]')
expect(script).toBeInTheDocument()
expect(script?.textContent).toContain('resolvedServerColorMode')
})
})

describe('primer_react_theme_provider_remove_ssr_handoff feature flag', () => {
it('does not render the script tag when the feature flag is enabled', () => {
const {container} = render(
<FeatureFlags flags={{primer_react_theme_provider_remove_ssr_handoff: true}}>
<ThemeProvider preventSSRMismatch>
<span>Hello</span>
</ThemeProvider>
</FeatureFlags>,
)

const script = container.querySelector('script[type="application/json"]')
expect(script).not.toBeInTheDocument()
})

it('does not render the script tag when the feature flag is enabled and contextOnly is true', () => {
const {container} = render(
<FeatureFlags flags={{primer_react_theme_provider_remove_ssr_handoff: true}}>
<ThemeProvider contextOnly preventSSRMismatch>
<span>Hello</span>
</ThemeProvider>
</FeatureFlags>,
)

const script = container.querySelector('script[type="application/json"]')
expect(script).not.toBeInTheDocument()
})

it('renders the script tag when the feature flag is disabled', () => {
const {container} = render(
<FeatureFlags flags={{primer_react_theme_provider_remove_ssr_handoff: false}}>
<ThemeProvider preventSSRMismatch>
<span>Hello</span>
</ThemeProvider>
</FeatureFlags>,
)

const script = container.querySelector('script[type="application/json"]')
expect(script).toBeInTheDocument()
expect(script?.textContent).toContain('resolvedServerColorMode')
})
})
2 changes: 1 addition & 1 deletion packages/react/src/utils/useTheme.hookDocs.json
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Expand Up @@ -33,7 +33,7 @@
{
"name": "preventSSRMismatch",
"type": "boolean",
"description": "Prevent SSR mismatch by injecting server-side color mode."
"description": "Deprecated: This prop is no longer used and has no effect."
}
]
},
Expand Down
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