Clean up experimental flags and turn some on

.eslintrc.js

@@ -521,6 +521,7 @@ module.exports = {
     spyOnProd: 'readonly',
     __DEV__: 'readonly',
     __EXPERIMENTAL__: 'readonly',
+    __NEXT_MAJOR__: 'readonly',
     __EXTENSION__: 'readonly',
     __PROFILE__: 'readonly',
     __TEST__: 'readonly',

packages/react-devtools-core/webpack.backend.js

@@ -70,7 +70,7 @@ module.exports = {
     }),
     new Webpack.DefinePlugin({
       __DEV__,
-      __EXPERIMENTAL__: true,
+      __EXPERIMENTAL__: 'readonly',
       __EXTENSION__: false,
       __PROFILE__: false,
       __TEST__: NODE_ENV === 'test',

packages/shared/ReactFeatureFlags.js

@@ -134,6 +134,38 @@ export const retryLaneExpirationMs = 5000;
 export const syncLaneExpirationMs = 250;
 export const transitionLaneExpirationMs = 5000;
 
+// -----------------------------------------------------------------------------
+// Ready for next major.
+//
+// __NEXT_MAJOR__ is an alias for __EXPERIMENTAL__.
+// -----------------------------------------------------------------------------
+
+// Not ready to break experimental yet.
+export const disableLegacyContext = __NEXT_MAJOR__;
+
+// Not ready to break experimental yet.
+// Disable javascript: URL strings in href for XSS protection.
+export const disableJavaScriptURLs = __NEXT_MAJOR__;
+
+// Remove IE and MsApp specific workarounds for innerHTML
+export const disableIEWorkarounds = __NEXT_MAJOR__;
+
+// Changes the behavior for rendering custom elements in both server rendering
+// and client rendering, mostly to allow JSX attributes to apply to the custom
+// element's object properties instead of only HTML attributes.
+// https://github.com/facebook/react/issues/11347
+export const enableCustomElementPropertySupport = __NEXT_MAJOR__;
+
+// Filter certain DOM attributes (e.g. src, href) if their values are empty
+// strings. This prevents e.g. <img src=""> from making an unnecessary HTTP
+// request for certain browsers.
+export const enableFilterEmptyStringAttributesDOM = __NEXT_MAJOR__;
+
+// Modern <StrictMode /> behaviour aligns more with what components
+// components will encounter in production, especially when used With <Offscreen />.
+// TODO: clean up legacy <StrictMode /> once tests pass WWW.
+export const useModernStrictMode = __NEXT_MAJOR__;
+
 // -----------------------------------------------------------------------------
 // Chopping Block
 //
@@ -150,8 +182,6 @@ export const createRootStrictEffectsByDefault = false;
 
 export const disableModulePatternComponents = false;
 
-export const disableLegacyContext = false;
-
 export const enableUseRefAccessWarning = false;
 
 // Enables time slicing for updates that aren't wrapped in startTransition.
@@ -173,29 +203,12 @@ export const allowConcurrentByDefault = false;
 // in open source, but www codebase still relies on it. Need to remove.
 export const disableCommentsAsDOMContainers = true;
 
-// Disable javascript: URL strings in href for XSS protection.
-export const disableJavaScriptURLs = false;
-
 export const enableTrustedTypesIntegration = false;
 
 // Prevent the value and checked attributes from syncing with their related
 // DOM properties
 export const disableInputAttributeSyncing = false;
 
-// Remove IE and MsApp specific workarounds for innerHTML
-export const disableIEWorkarounds = __EXPERIMENTAL__;
-
-// Filter certain DOM attributes (e.g. src, href) if their values are empty
-// strings. This prevents e.g. <img src=""> from making an unnecessary HTTP
-// request for certain browsers.
-export const enableFilterEmptyStringAttributesDOM = __EXPERIMENTAL__;
-
-// Changes the behavior for rendering custom elements in both server rendering
-// and client rendering, mostly to allow JSX attributes to apply to the custom
-// element's object properties instead of only HTML attributes.
-// https://github.com/facebook/react/issues/11347
-export const enableCustomElementPropertySupport = __EXPERIMENTAL__;
-
 // Disables children for <textarea> elements
 export const disableTextareaChildren = false;
 
@@ -245,8 +258,4 @@ export const enableProfilerNestedUpdateScheduledHook = false;
 
 export const consoleManagedByDevToolsDuringStrictMode = true;
 
-// Modern <StrictMode /> behaviour aligns more with what components
-// components will encounter in production, especially when used With <Offscreen />.
-// TODO: clean up legacy <StrictMode /> once tests pass WWW.
-export const useModernStrictMode = false;
 export const enableDO_NOT_USE_disableStrictPassiveEffect = false;

scripts/flow/environment.js

@@ -12,6 +12,7 @@
 declare var __PROFILE__: boolean;
 declare var __UMD__: boolean;
 declare var __EXPERIMENTAL__: boolean;
+declare var __NEXT_MAJOR__: boolean;
 declare var __VARIANT__: boolean;
 
 declare var __REACT_DEVTOOLS_GLOBAL_HOOK__: any; /*?{

scripts/jest/setupEnvironment.js

@@ -19,6 +19,8 @@ global.__EXPERIMENTAL__ =
     ? RELEASE_CHANNEL === 'experimental'
     : true;
 
+global.__NEXT_MAJOR__ = __EXPERIMENTAL__;
+
 global.__VARIANT__ = !!process.env.VARIANT;
 
 if (typeof window !== 'undefined') {

scripts/rollup/build.js

@@ -34,6 +34,8 @@ const __EXPERIMENTAL__ =
     ? RELEASE_CHANNEL === 'experimental'
     : true;
 
+const __NEXT_MAJOR__ = __EXPERIMENTAL__;
+
 // Errors in promises should be fatal.
 let loggedErrors = new Set();
 process.on('unhandledRejection', err => {
@@ -468,6 +470,7 @@ function getPlugins(
             ? "'production'"
             : "'development'",
           __EXPERIMENTAL__,
+          __NEXT_MAJOR__,
         },
       }),
       // The CommonJS plugin *only* exists to pull "art" into "react-art".