Function component renders incorrectly after Fast Refresh

I’ve encountered at least one scenario where a function component rendered after a Fast Refresh looks wrong and requires a hard refresh to fix. Changing it to a class component fixes this.

GIF:

React Native version:

System:
    OS: macOS 10.15.2
    CPU: (8) x64 Intel(R) Core(TM) i7-8559U CPU @ 2.70GHz
    Memory: 17.85 MB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 13.7.0 - /var/folders/7q/w_rts5kx5gzfk051cpw8vlzh0000gn/T/yarn--1579912407030-0.8849373494224462/node
    Yarn: 1.21.1 - /var/folders/7q/w_rts5kx5gzfk051cpw8vlzh0000gn/T/yarn--1579912407030-0.8849373494224462/yarn
    npm: 6.13.6 - ~/.nvm/versions/node/v13.7.0/bin/npm
  SDKs:
    iOS SDK:
      Platforms: iOS 13.2, DriverKit 19.0, macOS 10.15, tvOS 13.2, watchOS 6.1
  IDEs:
    Android Studio: 3.5 AI-191.8026.42.35.6010548
    Xcode: 11.3.1/11C505 - /usr/bin/xcodebuild
  npmPackages:
    react: ~16.9.0 => 16.9.0
    react-native: https://github.com/expo/react-native/archive/sdk-36.0.0.tar.gz => 0.61.4

Steps To Reproduce

1. Pull down this project: https://github.com/mxhold/rn-fast-refresh-bug
2. Open app in iOS simulator with Fast Refresh turned on.
3. Uncomment the commented line in App.tsx that sets flexGrow.
4. Save the file, triggering a refresh. Note the border is unchanged, as expected.
5. Comment out the same line (returning the file to its original contents).
6. Save the file again, triggering a refresh again. Note the border has collapsed as if flex: 1 was not set (!).
7. Press Cmd-R to refresh. Note the border goes back to the original position.

Describe what you expected to happen:

It is surprising that after step 6 the screen renders incorrectly and requires a hard refresh to render correctly.

I would have expected that after step 6, the border would have stayed in the same position but instead the view is rendered as if flex: 1 was not set at all.

Notes

• This example doesn’t work if you change App to a class component. It has to be a function component.
• This doesn’t seem to be reproducible in a Snack.

[gaearon]

@AndrewMorsillo This sounds quite bizarre! (I don’t think it has any relation to this issue btw so you should file a new one.)

[mxhold]

@gaearon