Debug render-phase side effects in "strict" mode (#12094)

A new feature flag has been added, debugRenderPhaseSideEffectsForStrictMode. When enabled, StrictMode subtrees will also double-invoke lifecycles in the same way as debugRenderPhaseSideEffects.

By default, this flag is enabled for __DEV__ only. Internally we can toggle it with a GK.

This breaks several of our incremental tests which make use of the noop-renderer. Updating the tests to account for the double-rendering in development mode makes them significantly more complicated. The most straight forward fix for this will be to convert them to be run as internal tests only. I believe this is reasonable since we are the only people making use of the noop renderer.

packages/shared/ReactFeatureFlags.js

@@ -26,6 +26,12 @@ export const enableNewContextAPI = false;
 // Helps identify side effects in begin-phase lifecycle hooks and setState reducers:
 export const debugRenderPhaseSideEffects = false;
 
+// In some cases, StrictMode should also double-render lifecycles.
+// This can be confusing for tests though,
+// And it can be bad for performance in production.
+// This feature flag can be used to control the behavior:
+export const debugRenderPhaseSideEffectsForStrictMode = __DEV__;
+
 // Warn about deprecated, async-unsafe lifecycles; relates to RFC #6:
 export const warnAboutDeprecatedLifecycles = false;
 

packages/shared/forks/ReactFeatureFlags.native-fabric.js

@@ -13,6 +13,7 @@ import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
 import typeof * as FabricFeatureFlagsType from './ReactFeatureFlags.native-fabric';
 
 export const debugRenderPhaseSideEffects = false;
+export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableAsyncSubtreeAPI = true;
 export const enableCreateRoot = false;
 export const enableUserTimingAPI = __DEV__;

packages/shared/forks/ReactFeatureFlags.native.js

@@ -15,6 +15,7 @@ import typeof * as FeatureFlagsShimType from './ReactFeatureFlags.native';
 // Re-export dynamic flags from the fbsource version.
 export const {
   debugRenderPhaseSideEffects,
+  debugRenderPhaseSideEffectsForStrictMode,
   warnAboutDeprecatedLifecycles,
 } = require('ReactFeatureFlags');
 

packages/shared/forks/ReactFeatureFlags.www.js

@@ -13,6 +13,7 @@ import typeof * as FeatureFlagsShimType from './ReactFeatureFlags.www';
 // Re-export dynamic flags from the www version.
 export const {
   debugRenderPhaseSideEffects,
+  debugRenderPhaseSideEffectsForStrictMode,
   warnAboutDeprecatedLifecycles,
 } = require('ReactFeatureFlags');