feat: support React 19 bigint, and deprecate fragment flattening

[amanabiy]

Description

Cloudscape components (ColumnLayout, Grid, and SpaceBetween) currently flatten React Fragments using react-keyed-flatten-children (v2.2.1), which depends on react-is (^18.2.0). React 19 renamed internal type elements, breaking fragment detection and flattening when using older versions of react-is.

Changes

• React 16-18: Uses react-keyed-flatten-children for backward compatibility (fragments still flattened)
• React 19+: Uses Children.toArray() directly (fragments NOT flattened, BigInt support added)

Tested by

• using our column-layout in different versions of react from 16 - 19 application locally.
• A simple sandbox test showing toArray behaves as we expect in react 19

Related links, issue #, if available: PptYApl47Z5P

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (9fc78ce) to head (6f56617).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4249   +/-   ##
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  Coverage   97.19%   97.19%           
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  Files         886      887    +1     
  Lines       26021    26038   +17     
  Branches     9433     9439    +6     
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+ Hits        25291    25308   +17     
  Misses        724      724           
  Partials        6        6           

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[jperals]

Since React.version is undocumented, should we make this line more failproof? And if checking the version fails, then fall back to the new behavior (i.e, not handling fragments, same as version >= 19)

[amanabiy]

Good point, I will do it like that.

[jperals]

Is there a possible case where this will throw an error? Let's say some conditional rendering logic where a child can be rendered as undefined or a number.

[amanabiy]

if the child is a number, bigint (in react 19), or string the key will be undefined. so we will be assigning undefined key. But it shouldn't throw an error.

[jperals]

And what about undefined?

Before the changes, the code was safe because typeof undefined is not 'object', therefore we render undefined according to the ternary operator, but after the changes, we will try to access a property of undefined, which throws an error.

[amanabiy]

You're right that accessing a property on undefined would throw an error. However, this is safe because both Children.toArray() and react-keyed-flatten-children filter out undefined, null, and boolean values.

Looking at the type signature of toArray it returns the following: toArray(children: ReactNode | ReactNode[]): Array<Exclude<ReactNode, boolean | null | undefined>>; The return type explicitly excludes undefined, null, and boolean. React's Children.toArray() implementation (test example). Under the hood, react-keyed-flatten-children also uses Children.toArray so they are filtered automatically, in both cases.

Here is another place where we also use similar code (line 85). That said, if you'd prefer more safe approach, we could add a guard and/or I can change the other implementation as well. What do you think?