[Flyout System] Make session logic truly opt-in

[tsullivan]

Summary

Makes the Flyout System truly opt-in by changing the session value default from inherit to never.

Closes elastic#9158.

Why are we making this change?

The default of inherit was problematic. It meant that a flyout with no session value added would automatically become a child if a session flyout is active. This would be a problem in Kibana, where all flyouts currently have no session value. Work would be needed to address the problem by adding session="start" on them to avoid possible bugs, which means the default logic planned would amount to a breaking change.

Screenshots #

Impact to users

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)
  • If the changes unblock an issue in a different repo, smoke tested carefully (see Testing EUI features in Kibana ahead of time)
• 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)

[tsullivan]

Replaced with. elastic#9163