Bug: React 19.2 flushSync flickers briefly

[Eliav2]

We’re seeing a production-only flicker regression for flushSync calls in React 19.2 that does not occur in React 19.1.

Demo Links

• React 19.1 (no flicker):
  https://deploy-preview-8--react-responsive-overflow-list.netlify.app/

  source PR

  

• React 19.2 (visible flicker):
  https://deploy-preview-10--react-responsive-overflow-list.netlify.app/

  source PR

  

  Note: performance throttling was enabled here to see flickering more clearly while resize

What Happens

The component relies on flushSync inside useResizeObserver:
https://github.com/Eliav2/react-responsive-overflow-list/blob/main/src/hooks/useResizeObserver.ts

On resize, we intentionally trigger synchronous layout updates using flushSync to prevent any intermediate visual state from appearing.

Behavior Difference

React 19.1:
flushSync blocks and applies the update immediately — the intermediate state never appears.

React 19.2:
React momentarily exposes the intermediate layout state even inside a flushSync block in production builds, resulting in a very short but noticeable flicker.

Additional Notes

• Flicker only appears in production builds of React 19.2.
• Both 19.1 and 19.2 do not flicker in local development mode.
• No code changes between demos—only the React version changed.

[Eliav2]

Updated the demo. Now it's a single container that shrinks so the 'bug' could be inspected as a single event in both deployments.

Here's Chrome inspection profiling

heres the bug apears:
react-19.2.json.gz

Here's it doesn't:
react-19.1.json.gz

It's quite clear from the profiling that:

• in react 19.1 - after Event: Click event, a sequence of rendering is synchronously happening within the same Task, exactly how it's supposed to be.

• in react 19.2 inspection, we can see that the actual rendering happens in a much later Task, exposing a paint state of unwanted outcome, in short, it apears as annoying flickering

This is also extremely hard to debug because it happens only in production builds.

but the trend is clear, 2 paints only in react 19.1 and 4 on react 19.2

at this point i think react flushSync is not doing what it suppose to do in react 19.2 (update the DOM synchronously).

This repro is quite minimal, and I think it should be relatively straightforward to validate whether it's a real bug in react(react-dom actually) or not

[Eliav2]

also updated demos build to unminify code and to include source maps to source files so it easier to debug prod builds.