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Certain fiber types may have a ref attached to them. The main ones are
HostComponent and ClassComponent. During the render phase, we check if a
ref was passed to it, and if so, we schedule a Ref effect: `markRef`.
Currently, we're not consistent about whether we call `markRef` in the
begin phase or the complete phase. For some fiber types, I found that
`markRef` was called in both phases, causing redundant work.
After some investigation, I don't believe it's necessary to call
`markRef` in both the begin phase and the complete phase, as long as you
don't bail out before calling `markRef`.
I though that maybe it had to do with the
`attemptEarlyBailoutIfNoScheduledUpdates` branch, which is a fast path
that skips the regular begin phase if no new props, state, or context
were passed. But if the props haven't changed (referentially — the
`memo` and `shouldComponentUpdate` checks happen later), then it follows
that the ref couldn't have changed either. This is true even in the old
`createElement` runtime where `ref` is stored on the element instead of
as a prop, because there's no way to pass a new ref to an element
without also passing new props. You might argue this is a leaky
assumption, but since we're shifting ref to be just a regular prop
anyway, I think it's the correct way to think about it going forward.
I think the pattern of calling `markRef` in the complete phase may have
been left over from an earlier iteration of the implementation before
the bailout logic was structured like it is today.
So, I removed all the `markRef` calls from the complete phase. In the
case of ScopeComponent, which had no corresponding call in the begin
phase, I added one.
We already had a test that asserted that a ref is reattached even if the
component bails out, but I added some new ones to be extra safe.
The reason I'm changing this this is because I'm working on a different
change to move the ref handling logic in `coerceRef` to happen in render
phase of the component that accepts the ref, instead of during the
parent's reconciliation.
DiffTrain build for [c820097716](https://github.com/facebook/react/commit/c820097716c3d9765bf85bf58202a4975d99e450)