fix(ios): Fix rounding error occurring randomly in iOS text line height calculation

[soutua]

Summary:

Fixes issue #53450

There are some discussion and explanation about the issue here: #53450 (comment)

In short, there seems to be a rounding error occurring with the new architecture's implementation when the text paragraph line heights are calculated on iOS, that happens randomly and depends on what text content the view has, and on the device screen.

The fix is basically the same that had been implemented in the React Native's old architecture 8 years ago, but hasn't been brought into the new architecture implementation. The fix adds a fraction to the text line dimensions to mitigate the rounding error.

Changelog:

[IOS] [FIXED] - Fix rounding error occurring randomly in iOS text line height calculation

Test Plan:

Note that the reproduction of the issue is also dependant on the used device/simulator, this was tested using the iPhone 17 simulator.

• Go to Text Layout Example in the RNTester app
• Scroll down the text until you see this text paragraph:

Without the fix:

With the fix:

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[cipolleschi]

@soutua thanks for the fix!

[meta-codesync]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this in D85438723.

[emaestre]

Hey guys! Thank you so much for the fix 🚀 is there any timeline to get this released? I think that a lot of apps are having this trouble.

Thanks!

[cipolleschi]

Hey @soutua
Can I ask you to:

1. rebase
2. add an example screen in RNTester
3. show the before and after in the TestCase. So basically we want to see the problem before the fix and the problem fixed after the fix.

@emaestre the steps are:

1. The PR needs to be merged
2. if it is merged before friday of the next week, it will be released in 0.83
3. if you guys needs this fix in a previous version of react native, we need a cherry pick request
4. After the commit is picked, it will be included in the next patch release of the version.

[soutua]

Hey @soutua Can I ask you to:

1. rebase
2. add an example screen in RNTester
3. show the before and after in the TestCase. So basically we want to see the problem before the fix and the problem fixed after the fix.

I've now rebased and added a new text layout example screen in the RNTester app, I've also updated the test plan in the issue description to showcase the before/after difference.

[emaestre]

soutua

Thank you so much @cipolleschi!

I created a Cherry Pick request here: reactwg/react-native-releases#1161 (comment)

Appreciate the information and the help.

[soutua]

Noticed that the flow type check failed in the CI, I pushed new changes and it passes at least locally for me now, there was no types used in the test screen before this.

[soutua]

@cipolleschi Is it still possible to get this into the 0.83? I think you might have tried importing this last week but it failed in CI type checks, which I then fixed (I believe) after I got the notification.

I'm just thinking that if this doesn't make it to the 0.83, and considering e.g. Expo doesn't seem to support the even numbered React Native releases, then it might in practice be version 0.85 / Expo 56 that would have the fix for most people, which might be next fall or something 😅

[cipolleschi]

sure, we will need a cherry pick request, but this is a fix, so that's fine.

[meta-codesync]

@cipolleschi merged this pull request in 581d643.

[react-native-bot]

This pull request was successfully merged by @soutua in 581d643

^{When will my fix make it into a release? | How to file a pick request?}

[facebook-github-bot]

This pull request has been reverted by 6355501.

[Blackfaded]

This PR is now reverted? So the fix is not going to land in the next release?
I made a patch file and the fix seems the only way to get our news app working again... This is a mandatory fix.

[emaestre]

Hey guys! @cipolleschi @soutua @douglowder

Hope everyone is doing well. Any update on this? We're still experiencing the issue, and we would like to know what we can do, at least a workaround, while this is fixed.

[Blackfaded]

Hey guys! @cipolleschi @soutua @douglowder

Hope everyone is doing well. Any update on this? We're still experiencing the issue, and we would like to know what we can do, at least a workaround, while this is fixed.

You can use this to create a patchfile
https://github.com/facebook/react-native/pull/54260/files

Here is ours:
react-native+0.81.4.patch

diff --git a/node_modules/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm b/node_modules/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm
index 216bb23..b19b566 100644
--- a/node_modules/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm
+++ b/node_modules/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm
@@ -386,9 +386,9 @@ - (TextMeasurement)_measureTextStorage:(NSTextStorage *)textStorage
     size.height = enumeratedLinesHeight;
   }

-  size = (CGSize){
-      ceil(size.width * layoutContext.pointScaleFactor) / layoutContext.pointScaleFactor,
-      ceil(size.height * layoutContext.pointScaleFactor) / layoutContext.pointScaleFactor};
+  CGFloat epsilon = 0.001;
+  size = (CGSize){ceil((size.width + epsilon) * layoutContext.pointScaleFactor) / layoutContext.pointScaleFactor,
+                  ceil((size.height + epsilon) * layoutContext.pointScaleFactor) / layoutContext.pointScaleFactor};

   __block auto attachments = TextMeasurement::Attachments{};
   

[soutua]

Yep, you can patch it locally in your project. I don't know why the change was reverted from the main branch, probably something didn't work because of it, or alternatively it was reverted by mistake. Don't know if it's coming back or not.

[KestasVenslauskas]

So is this going to be released or not?

[Blackfaded]

I cant get why this is so hardcore ignored. This is a major bug within a most basic component.

[Kitcheone]

Agreed, especially as the fix provided by @soutua matches the fix that was in the old architecture. Details on what it broke would be good, so we could address them and get the fix merged.

[douglowder]

The original issue does seem like a pretty bad bug. The revert happened in #54510 because this change broke internal tests. @cipolleschi it would be great to understand what tests broke -- maybe there are certain values of size and pointScaleFactor where the result is off by one. I'll take a look to see if I (or maybe Claude 😄 ) can write some detailed unit tests for many different combinations of inputs.

[fabriziocucci]

FYI folks, we are trying to land the fix again in #57698.