[SG-36350] Accessibility: Global `Menu`: Focus isn't correctly captured when menus are opened

Created by: gitstart-sourcegraph

Description:

• Update Popover implementation to stop removing MenuItem from DOM but keep it invisible when menu is not opened.
• Update DOM and menu styles in :focus-visible state

Refs

SourceGraph Issue GitStart ticket

Test plan

• Check on UI to make sure the first menu item is selected by default when menu is opened
• Keyboard focusing menu item has the common outline style

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.