The Portal part cannot be omitted safely when nesting popups

[atomiks]

Omitting the Portal part can currently lead to various bugs when dealing with nested popups. The docs always use the Portal part, but omitting it is technically allowed right now (no runtime error).

Known bugs

Some of the bugs include (non-exhaustively):

• Nesting Popover inside a Dialog when neither has a portal prevents the esc key from only dismissing the Popover — both get dismissed simultaneously
• Nesting Popover, Select or Menu inside Dialog when only Dialog has a portal causes focus to be lost when attempting to tab outside of the inner popup's last tabbable element
• Nesting Menu inside Dialog when only the the root menu has a portal (with submenus not using a portal) causes the root Menu to stay stuck open when tabbing outside of one of the submenus

Some bugs are already covered in #1107, which has a PR open that fixes it already.

Solution

1. Throw a runtime error if Portal is missing entirely (this is likely the best solution, at least for now)
2. And/or, try to solve most of the bugs depending on the portal combination if possible, and document when omitting it won't work