Bug: Suspense should hide Portals deeper in the tree

When Portal is a direct child of Suspense, suspending hides the portal: https://codesandbox.io/s/cocky-boyd-mlq2ko?file=/src/App.js But when a Portal is deeper in the tree, suspending fails to hide the portal nodes: https://codesandbox.io/s/nostalgic-fog-udyhuz?file=/src/App.js We need to fix this to recursively hide portals. There's a question of which traversal to use, and how it combines with existing traversal. E.g. does "hiding" happen before layout effects run? We would also need to decide what to do with legacy mode. One option is to "fix" it there too. But that might be difficult to roll out. It might also be tricky to implement. Another option is to leave it as is (the current behavior is leaving a "hole" in place of the suspended component: https://codesandbox.io/s/elastic-ptolemy-2u6qel?file=/src/App.js). Then we'd need to make sure we at least keep that buggy behavior. We might want to add an internal-only warning (to be muted but logged on devservers) to track _newly hidden_ nodes portals that wouldn't have been hidden with the previous algorithm. Then maybe this would let us know where to fix the UI and avoid the UI regression caused by modals disappearing. Summary of work that needs to be done: - [ ] Port sandboxes to failing tests - [ ] Implement new behavior behind a flag - [ ] Figure out where hiding/showing happens during the commit phase - [ ] Add new behavior (all direct children of portals, no matter how deep, get auto-shown/hidden) - [ ] Decide what legacy renderer should do, write tests for that too - [ ] (Maybe) add internal-only muted logging for content that would be hidden with new behavior