Navigation: z-index issue with overlay in mobile devices [#3447223]

Problem/Motivation

Z-index issue with overlay in mobile devices. When data-admin-toolbar is expanded in mobile screen the overlay z-index variable is undefined.

Steps to reproduce

1. Enable navigation module
2. go to /admin/config/people/accounts in mobile screen
3. Expand the data-admin-toolbar
4. Now see the overlay conflicts with screen elements. please see screenshot for reference.

Proposed resolution

Add z-index variable

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet

Comment	File	Size	Author
	Screenshot 2024-05-15 at 9.07.44 AM.png	126.81 KB	gauravvvv

Issue fork drupal-3447223

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3447223-navigation-z-index changes, plain diff MR !8079
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