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[flags] add enableEffectEventMutationPhase #35548

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@rickhanlonii rickhanlonii commented Jan 17, 2026 •
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Small optimization for useEffectEvent. Not sure we even need a flag for it, but it will be a nice killswitch.

As an added benefit, it fixes a bug when enableViewTransition is on, where we were not updating the useEffectEvent callback when a tree went from hidden to visible.

@meta-cla meta-cla bot added the CLA Signed label Jan 17, 2026
@github-actions github-actions bot added the React Core Team Opened by a member of the React Core Team label Jan 17, 2026
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react-sizebot commented Jan 17, 2026 •
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Comparing: cdbd55f...cd267f4

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name +/- Base Current +/- gzip Base gzip Current gzip
oss-stable/react-dom/cjs/react-dom.production.js = 6.84 kB 6.84 kB +0.05% 1.88 kB 1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js = 608.67 kB 608.48 kB = 107.63 kB 107.60 kB
oss-experimental/react-dom/cjs/react-dom.production.js = 6.84 kB 6.84 kB +0.05% 1.88 kB 1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js = 674.60 kB 670.95 kB = 118.57 kB 118.02 kB
facebook-www/ReactDOM-prod.classic.js +0.05% 694.04 kB 694.39 kB +0.09% 122.01 kB 122.12 kB
facebook-www/ReactDOM-prod.modern.js +0.05% 684.43 kB 684.77 kB +0.10% 120.40 kB 120.51 kB
oss-experimental/react-server/cjs/react-server.development.js = 214.39 kB 209.99 kB = 37.90 kB 37.10 kB
oss-stable-semver/react-server/cjs/react-server.development.js = 209.60 kB 205.20 kB = 36.97 kB 36.17 kB
oss-stable/react-server/cjs/react-server.development.js = 209.60 kB 205.20 kB = 36.97 kB 36.17 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show
Name +/- Base Current +/- gzip Base gzip Current gzip
oss-experimental/react-reconciler/cjs/react-reconciler-reflection.development.js +0.71% 12.75 kB 12.84 kB +0.78% 2.71 kB 2.73 kB
oss-stable-semver/react-reconciler/cjs/react-reconciler-reflection.development.js +0.71% 12.75 kB 12.84 kB +0.78% 2.71 kB 2.73 kB
oss-stable/react-reconciler/cjs/react-reconciler-reflection.development.js +0.71% 12.75 kB 12.84 kB +0.78% 2.71 kB 2.73 kB
oss-experimental/react-reconciler/cjs/react-reconciler-reflection.production.js +0.59% 11.29 kB 11.36 kB +0.65% 2.63 kB 2.65 kB
oss-stable-semver/react-reconciler/cjs/react-reconciler-reflection.production.js +0.59% 11.29 kB 11.36 kB +0.65% 2.63 kB 2.65 kB
oss-stable/react-reconciler/cjs/react-reconciler-reflection.production.js +0.59% 11.29 kB 11.36 kB +0.65% 2.63 kB 2.65 kB
oss-experimental/react-noop-renderer/cjs/react-noop-renderer-persistent.development.js = 42.94 kB 42.82 kB = 7.78 kB 7.75 kB
oss-stable/react-noop-renderer/cjs/react-noop-renderer-persistent.development.js = 42.94 kB 42.81 kB = 7.77 kB 7.75 kB
oss-stable-semver/react-noop-renderer/cjs/react-noop-renderer-persistent.development.js = 42.91 kB 42.79 kB = 7.75 kB 7.72 kB
oss-experimental/react-noop-renderer/cjs/react-noop-renderer.development.js = 42.80 kB 42.68 kB = 7.76 kB 7.73 kB
oss-stable/react-noop-renderer/cjs/react-noop-renderer.development.js = 42.80 kB 42.67 kB = 7.76 kB 7.73 kB
oss-stable-semver/react-noop-renderer/cjs/react-noop-renderer.development.js = 42.77 kB 42.65 kB = 7.73 kB 7.70 kB
oss-experimental/react-noop-renderer/cjs/react-noop-renderer-persistent.production.js = 38.62 kB 38.50 kB = 7.17 kB 7.15 kB
oss-stable/react-noop-renderer/cjs/react-noop-renderer-persistent.production.js = 38.61 kB 38.49 kB = 7.17 kB 7.14 kB
oss-stable-semver/react-noop-renderer/cjs/react-noop-renderer-persistent.production.js = 38.58 kB 38.47 kB = 7.14 kB 7.11 kB
oss-experimental/react-noop-renderer/cjs/react-noop-renderer.production.js = 38.49 kB 38.37 kB = 7.15 kB 7.13 kB
oss-stable/react-noop-renderer/cjs/react-noop-renderer.production.js = 38.48 kB 38.37 kB = 7.15 kB 7.12 kB
oss-stable-semver/react-noop-renderer/cjs/react-noop-renderer.production.js = 38.46 kB 38.34 kB = 7.12 kB 7.09 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js = 1,266.92 kB 1,262.88 kB = 210.58 kB 209.88 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js = 1,266.92 kB 1,262.87 kB = 211.36 kB 210.67 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js = 1,250.37 kB 1,246.33 kB = 207.72 kB 207.01 kB
oss-experimental/react-art/cjs/react-art.development.js = 736.63 kB 734.08 kB = 115.79 kB 115.50 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js = 865.76 kB 862.57 kB = 134.78 kB 134.34 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js = 753.50 kB 750.23 kB = 130.10 kB 129.58 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js = 689.01 kB 685.36 kB = 122.08 kB 121.56 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js = 674.60 kB 670.95 kB = 118.57 kB 118.02 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js = 571.41 kB 568.03 kB = 88.92 kB 88.36 kB
oss-experimental/react-markup/cjs/react-markup.react-server.development.js = 672.86 kB 668.57 kB = 119.15 kB 118.34 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js = 495.52 kB 491.86 kB = 78.71 kB 78.14 kB
oss-experimental/react-art/cjs/react-art.production.js = 361.64 kB 358.82 kB = 60.84 kB 60.39 kB
oss-experimental/react-dom/cjs/react-dom-server.node.development.js = 460.87 kB 456.48 kB = 80.35 kB 79.49 kB
oss-experimental/react-dom/cjs/react-dom-server.edge.development.js = 455.11 kB 450.72 kB = 80.59 kB 79.73 kB
oss-experimental/react-dom/cjs/react-dom-server.browser.development.js = 454.11 kB 449.71 kB = 80.36 kB 79.50 kB
oss-stable/react-dom/cjs/react-dom-server.node.development.js = 444.86 kB 440.47 kB = 78.09 kB 77.21 kB
oss-stable-semver/react-dom/cjs/react-dom-server.node.development.js = 444.79 kB 440.39 kB = 78.04 kB 77.16 kB
oss-experimental/react-dom/cjs/react-dom-server.bun.development.js = 402.04 kB 398.06 kB = 75.90 kB 75.08 kB
oss-stable/react-dom/cjs/react-dom-server.edge.development.js = 438.60 kB 434.21 kB = 78.29 kB 77.42 kB
oss-stable-semver/react-dom/cjs/react-dom-server.edge.development.js = 438.53 kB 434.13 kB = 78.24 kB 77.37 kB
oss-stable/react-dom/cjs/react-dom-server.browser.development.js = 437.82 kB 433.43 kB = 78.10 kB 77.25 kB
oss-stable-semver/react-dom/cjs/react-dom-server.browser.development.js = 437.75 kB 433.35 kB = 78.05 kB 77.20 kB
oss-stable/react-dom/cjs/react-dom-server.bun.development.js = 387.88 kB 383.90 kB = 73.59 kB 72.79 kB
oss-stable-semver/react-dom/cjs/react-dom-server.bun.development.js = 387.81 kB 383.83 kB = 73.56 kB 72.76 kB
oss-experimental/react-dom/cjs/react-dom-server-legacy.browser.development.js = 423.72 kB 419.33 kB = 76.14 kB 75.32 kB
oss-experimental/react-dom/cjs/react-dom-server-legacy.node.development.js = 423.72 kB 419.32 kB = 76.14 kB 75.32 kB
facebook-www/ReactDOMServer-dev.classic.js = 422.74 kB 418.34 kB = 75.89 kB 75.05 kB
facebook-www/ReactDOMServer-dev.modern.js = 419.30 kB 414.90 kB = 75.29 kB 74.45 kB
facebook-www/ReactDOMServerStreaming-dev.modern.js = 417.81 kB 413.41 kB = 74.83 kB 74.02 kB
oss-stable/react-dom/cjs/react-dom-server-legacy.browser.development.js = 408.43 kB 404.03 kB = 73.92 kB 73.10 kB
oss-stable/react-dom/cjs/react-dom-server-legacy.node.development.js = 408.43 kB 404.03 kB = 73.92 kB 73.10 kB
oss-stable-semver/react-dom/cjs/react-dom-server-legacy.browser.development.js = 408.40 kB 404.01 kB = 73.90 kB 73.07 kB
oss-stable-semver/react-dom/cjs/react-dom-server-legacy.node.development.js = 408.40 kB 404.01 kB = 73.90 kB 73.07 kB
oss-experimental/react-markup/cjs/react-markup.development.js = 395.45 kB 391.18 kB = 71.81 kB 71.05 kB
oss-experimental/react-server/cjs/react-server.development.js = 214.39 kB 209.99 kB = 37.90 kB 37.10 kB
oss-stable-semver/react-server/cjs/react-server.development.js = 209.60 kB 205.20 kB = 36.97 kB 36.17 kB
oss-stable/react-server/cjs/react-server.development.js = 209.60 kB 205.20 kB = 36.97 kB 36.17 kB

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@rickhanlonii rickhanlonii mentioned this pull request Jan 18, 2026
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rickhanlonii commented Jan 18, 2026
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packages/react-reconciler/src/__tests__/useEffectEvent-test.js Outdated
});

// @gate enableActivity
it('effect events are fresh in deeply nested hidden Activities', async () => {
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@sebmarkbage this is a simplified test.

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