TextInput is unstable for Korean. (iOS only)

[toy0605]

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Description

Hi.
I found the issue on iOS devices.
I'm typing with Korean on TextInput. but font size changes madly or flickering during typing.
It's fine for english. It only has issue for Korean or mixing english and korean.

It causes from two ways.

1. Font family is not assigned.
2. Font family is Korean font.(NotoSansKR or Apple SD Gothic Neo) It looks good this way. but If add props like value and event handler(onChangeText) for changing value, it makes unstable, too.

It only works very well functionally. appearance is so bad.

That issue very annoying to me.

React Native version:

System:
 OS: macOS 10.15.5
 CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
 Memory: 738.23 MB / 32.00 GB
 Shell: 5.7.1 - /bin/zsh
 Binaries:
 Node: 14.0.0 - /usr/local/bin/node
 Yarn: 1.22.4 - /usr/local/bin/yarn
 npm: 6.14.4 - /usr/local/bin/npm
 Watchman: 4.9.0 - /usr/local/bin/watchman
 Managers:
 CocoaPods: 1.9.2 - /usr/local/bin/pod
 SDKs:
 iOS SDK:
 Platforms: iOS 13.5, DriverKit 19.0, macOS 10.15, tvOS 13.4, watchOS 6.2
 Android SDK: Not Found
 IDEs:
 Android Studio: 3.6 AI-192.7142.36.36.6392135
 Xcode: 11.5/11E608c - /usr/bin/xcodebuild
 Languages:
 Java: 1.8.0_252 - /usr/bin/javac
 Python: 2.7.16 - /usr/bin/python
 npmPackages:
 @react-native-community/cli: Not Found
 react: 16.11.0 => 16.11.0
 react-native: 0.62.2 => 0.62.2
 npmGlobalPackages:
 *react-native*: Not Found

Steps To Reproduce

The issue for TextInput only. Not Text component.

1. Font family is none.
2. Font family is Korean font. but using props(value and onChangeText)

and typing Korean text.

Expected Results

I just want it looks stable.

Snack, code example, screenshot, or link to a repository:

<TextInput style={{width: "100%", backgroundColor: "#eeeeee", fontSize: 14}}/>

<TextInput style={{width: "100%", backgroundColor: "#eeeeee", fontFamily: "NotoSansKR-Regular", fontSize: 14}}
value={this.state.textValue}
onChangeText={(text) => this.setState({textValue: text})}/>

[toy0605]

If multiline is true, it works very well strangely.

[javascripter]

This happens with Japanese too.

When there is some English character before Japanese, the font size stays stable. Otherwise it causes significant layout instability.

[firevitality]

Chinese without English too.

[mengheangrat]

It's RN issue. You can use this patch.
https://github.com/ifsnow/react-native-cjk-textinput-patch#readme

[chrisglein]

Fixed by #28805

[wbw20]

I tested this fix on #8c392ff and it seems to work well for me, but it seems to be broken on 0.63.0.