Problem with rendering child component on props changes

[Sergey Kravchenko]

Describe the bug
There was a problem with the rendering of components when updating props.
If you pass the state of a parent component as a prop to a child component, and then update the state in the parent component, the child component is not rerendered.

To Reproduce
Preact - https://codepen.io/coder4u/pen/YzgdQEL

React without rerender issue - https://codepen.io/coder4u/pen/qBvLXOy

Steps to reproduce the behavior:

1. Go to pen.
2. Open console.
3. You will see that parent state not synced with child props in render methods.

Expected behavior
Child component must rerender when it props changed.

[Jovi De Croock]

Looks like this got broken between 10.18.1 and 10.18.2, wondering whether this contributed to it. Will investigate.

EDIT: this looks to be a timing issue as it's not really reproducible in our test suite 😅

[dmitrage]

Did some debugging, maybe it could help.

When BrokenChild is rendered for the first time its vnode._original == 3 (++vnodeId).
After setState in BrokenChild.componentDidUpdate it increments to 4. Note that global vnodeId == 3 at this time.
When App component re-renders after setTimeout it creates a new vnode for BrokenChild with _original == 4 (++vnodeId).
Then rendering bails out because oldVNode._original == newVNode._original.

Before the mentioned change _original was changed on a temporary copy of the oldVNode but now it is changed on the newVNode.

[Jovi De Croock]

Yeah, I got to a similar conclusion, we might just need to change the vnodeId back after diffing 😅