[Fiber] Fix TestUtils edge cases

[gaearon]

• Fixes an issue where functional components were ignored when "scrying"
• Fixes an issue where we looked at work in progress instead of the current node

The rest of TestUtils failures are now all Simulate related.

[gaearon]

I'm not sure if this is a good way to tell that we're in a current tree. Any pitfalls?

[acdlite]

I'm facing the same problem in the PR I'm working on.

[acdlite]

👍 but let's decide (or clarify) the established way to determine whether a fiber is current

[acdlite]

Hmm, I don't think following the return path guarantees that a fiber is current. They aren't parallel trees: #7344 (comment)

[gaearon]

What's the right way to get the current version of a given Fiber? I tried looking at ReactFiberReflection but it searches for host parents whereas I want to search for the current version of this fiber.

[acdlite]

I think isFiberMounted is what you want. It searches for host parents because if a fiber is current then it follows that it has a HostContainer parent.

[acdlite]

So maybe

function getCurrentFiber(fiber : Fiber): Fiber | null {
  if (isFiberMounted(fiber)) {
    return fiber;
  } else if (fiber.alternate && isFiberMounted(fiber.alternate)) {
    return fiber.alternate;
  } else {
    return null;
  }
}

[gaearon]

I tried this but isFiberMounted is true for both of them.

[acdlite]

Huh interesting. Maybe what you did is correct then. Can't say I fully understand, though.

Could you put your solution into ReactFiberTreeReflection? I need it in my PR as well. If it's wrong in some way we can fix it later.

[acdlite]

Actually I think I understand now. Your solution is correct 👍

[gaearon]

Haha now I'm not sure if it's correct. 😄

[gaearon]

I think #7344 (comment) doesn't apply because I don't care if either node on the path is current or not. I only care about the root node for which I can find it out. I also care that if a node's root is current, it follows that the node is current. Does this assumption always hold?

[acdlite]

I think it does because the return field is always set during reconciliation in ReactChildFiber

[gaearon]

OK, let's do it and if it's broken then add a breaking test.