What

Render custom overlay template parts assigned to a Navigation block on the front end using the existing overlay interactivity system. This ensures custom overlays inherit established show/hide behaviours, animations, and accessibility controls without introducing new logic or regressions.

Why

Front-end rendering is essential for custom overlays to function beyond the editor. Reusing the existing overlay system ensures consistent behaviour, performance, and accessibility across themes. Addressing edge cases - such as missing or unpublished overlays, or overlays containing additional Navigation blocks - is critical for providing a stable, accessible experience that meets user and search engine expectations.

How

Update the Navigation block’s rendering logic to:

• Output the selected custom overlay template part using the existing overlay interactivity structure.
• Ensure parity with the current behaviour for toggling, focus management, and animation.
• Handle missing or unpublished overlays gracefully with clear fallbacks or warnings.
• Audit and address accessibility and SEO implications when overlays contain nested Navigation blocks or additional interactive content.

This issue depends on overlay creation (#73079) and overlay selection (#73080).