reference: https://github.com/vuejs/rfcs/issues/121
BREAKING CHANGE: object returned from `setup()` are no longer implicitly
passed to `reactive()`.
The renderContext is the object returned by `setup()` (or a new object
if no setup() is present). Before this change, it was implicitly passed
to `reactive()` for ref unwrapping. But this has the side effect of
unnecessary deep reactive conversion on properties that should not be
made reactive (e.g. computed return values and injected non-reactive
objects), and can lead to performance issues.
This change removes the `reactive()` call and instead performs a
shallow ref unwrapping at the render proxy level. The breaking part is
when the user returns an object with a plain property from `setup()`,
e.g. `return { count: 0 }`, this property will no longer trigger
updates when mutated by a in-template event handler. Instead, explicit
refs are required.
This also means that any objects not explicitly made reactive in
`setup()` will remain non-reactive. This can be desirable when
exposing heavy external stateful objects on `this`.
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