[Overlay] Review management of overlay stack in relationship to a new stack root.

[Westbrook]

Code of conduct

• I agree to follow this project's code of conduct.

Description of issue

Clarify usage documentation and/or API surface of the Overlay API around the support of multiple "overlay roots". This comes into play when two sibling overlay triggers exist and one of the triggers have a complex trail of overlays open without using type="modal" to occlude interaction with the second sibling. How should the library/a user manage interactions with the second trigger?

Context:

• visit https://webcomponents.dev/edit/d7PlCqV4TsBtHJWbRdqy/src/index.ts
• open the "Complex Popover"
• open the Picker in that popover
• attempt to open the "Second Popover"

Questions:

• should this close the first popover?
  • always
  • sometimes
  • never
• Complex Popover[type="modal"] prevents interaction with "Second Popover" until closed, should this be the suggested usage?
  • modal traps tab
  • modal currently passes contextmenu interaction to the page once the popover is closed, should more things do that?
• The stack:
  • The overlay system is currently a single stack where we generally push and pop the last item, should there be there be a deeper concept of "groups" or similar in the stack that close all at once?
  • Should "groups" be a usage location concept that a developer needs to manage in some way themselves?
  • This come to bear in the concept of sub/flyout menus, as well:
    • Visit https://sub-menu--spectrum-web-components.netlify.app/storybook/?path=/story/menu--submenu
    • use the pointer to expand multiple levels of sub menu
    • click and option in the deepest submenu
    • see all sub menus close
    • is this a "group"?

cc: @spdev3000 @jstoeckm

[Westbrook]

Closing in favor of #2086