Fix getBoundingClientRect() performance with <legend> elements.

[WOLFRIEND]

Description

Fixes #39428.
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Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

https://stackblitz.com/edit/js-dydeas?file=index.html

Related issues

• Flex container doesn't display correctly next to a legend #29712.