[Visual Refresh] Update floating border on EuiPanel

[mgadewoll]

Summary

Important

This PR merges into a feature branch.

This PR updates the implementation of the floating border on EuiPanel to fix the issue of the transparent border on light mode showing with colored full-width content (e.g. for EuiSplitPanel)

Instead of adding the floating border on the panel element directly, this PR adds a pseudo element for the border.
This will:

• prevent transparent borders from showing as the border overlaps the content instead of sitting around the content
• ensures layout parity between light and dark mode (having a conditional border on dark mode would otherwise mean that light and dark mode have slightly different dimensions for panels as borders add to the size)

before	after

QA

Note

To enable Borealis in EUI PR deployments, you'll need to:

• add the following localStorage item: eui-experimental-theme-enabled: true
• reload the page
• select Borealis from the theme switcher

• verify the floating border on EuiSplitPanel (Storybook, EUI docs) is visible for Borealis only
• verify the border does not affect dimensions between light and dark mode
• verify panels look the same between production and staging

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8270/
• Storybook: https://eui.elastic.co/pr_8270/storybook
• New docs: https://eui.elastic.co/pr_8270/new-docs

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 11d4754

History

• 💚 Build #3315 succeeded 2168cd7

cc @mgadewoll

[weronikaolejniczak]

LGTM 👍🏻 Great job, Lene!

Testing

Storybook (Borealis)

Screen.Recording.2025-01-14.at.12.01.33.mov

Storybook (Amsterdam)

Screen.Recording.2025-01-14.at.12.05.00.mov

EUI Docs (Borealis)

Screen.Recording.2025-01-14.at.12.03.45.mov

EUI Docs (Amsterdam)

Screen.Recording.2025-01-14.at.12.06.13.mov

Screen.Recording.2025-01-14.at.12.15.02.mov

Production vs Staging (Amsterdam)

Screen.Recording.2025-01-14.at.12.06.13.mov

Screen.Recording.2025-01-14.at.12.13.09.mov