fix: incorrect z-index in fast-menu slot='submenu'

[robarbms]

Pull Request

📖 Description

When using a menu next to another menu, submenus will appear under menu items of the next menu.

🎫 Issues

#5417

👩‍💻 Reviewer Notes

This is the least obtrusive way to get flyout menus to go over other elements. This will fix both the issue of a menu next to another menu as well as submenus going under an element next to it that has position: relative or position: absolute. Even if that element doesn't have a z-index property. Developers can still manage z-index on the fast-menu-item as in the example that I added to the fixture. They could also add z-index to the fast-menu but that becomes trickier if they are using menus side-by-side.

📑 Test Plan

Not really testable in Jest. Added a working example in the fixtures.

✅ Checklist

General

• I have included a change request file using $ yarn change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

⏭ Next Steps