Summary

Previously, when EuiFlyout used the session=true setting, it required a size prop to be passed. This PR makes the size prop optional, and defaults to m when it is not passed.

Why are we making this change?

This change will ease the migration of "plain" flyouts in Kibana to use the session=true setting.

Impact to users

Developers consuming EuiFlyout won't be forced to add a size prop to migrate existing "plain" flyouts to use the session=true setting.

QA

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General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in both MacOS and Windows high contrast modes
    • (emulate forced colors if you do not have access to a Windows machine.)
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
  • Props have proper autodocs (using @default if default values are missing) and playground toggles
  • Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
  • Updated visual regression tests
• Release checklist
  • A changelog entry exists and is marked appropriately.
  • If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist
  • If applicable, file an issue to update EUI's Figma library with any corresponding UI changes. (This is an internal repo, if you are external to Elastic, ask a maintainer to submit this request)