fix(tooltip): improve z-index calculations

[markov00]

Summary

Fixes tooltip portal stacking when charts are rendered inside higher z-index containers, such as Kibana flyouts or sidebars.

The previous z-index lookup walked the offsetParent chain, which can skip DOM ancestors that still define the relevant body-level stacking order. In Kibana, this meant the chart inside kbnChromeLayoutSidebar could compute a z-index of 0 instead of the sidebar level, so the tooltip portal stayed behind the flyout chrome.

This change:

• Walks DOM parents when computing the chart’s effective z-index, so ancestors like flyout/sidebar containers are considered.
• Updates the z-index on an existing portal node when it is reused, instead of only setting it on initial creation.

Test Plan

• Verified against a local Kibana flyout containing Elastic Charts.
• Confirmed the old offsetParent lookup returned 0, while the DOM parent lookup finds the sidebar z-index (1050).

Screen.Recording.2026-05-22.at.17.21.18.mp4

Issues

fix #2838

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• All related issues have been linked (i.e. closes #123, fixes #123)
• The code has been checked for cross-browser compatibility (Chrome, Firefox, Safari, Edge)

[elastic-datavis]

✅ Successful Deployment (build#396) - 3484873

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• e2e server
• Playwright VRT report
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• Packages - pr.tgz - 3484873.tgz