10.18.2 regression: Cannot read properties of undefined

[zakstucke]

• Check if updating to the latest Preact version resolves the issue

Describe the bug
Our dashboard starts fails when upgrading from 18.1 to 18.2. Looks like suspense boundary regressions.
I haven't got an easy repro or capacity to find one right now, just opening for visibility on the issue.

[JoviDeCroock]

@zakstucke any luck on the reproduction yet?

[zakstucke]

@JoviDeCroock sad to say not yet! Like with the last suspense issue I helped with a while back, they seem to pop up in very specific circumstances and so all seems to work fine in a simple repo...

I've just checked with the latest version: 10.19.3 where issue still persists, last working version 10.18.1.

[zakstucke]

Although looking at this new screenshot errs seem slightly different, now in signals, although this might be unrelated and from changes in my code.

[JoviDeCroock]

@zakstucke I wonder if it's https://github.com/preactjs/preact/pull/4182/files we should check for the existence of oldVNode where does the stacktrace lead you in preact? or it has something to do with _nextDom setting with the insertBefore warning... not sure

[zakstucke]

@JoviDeCroock @marvinhagemeister sorry to say that fix wasn't it!

But I've been useless for long enough, I've spent some time this morning trimming code until I got to a simple repro.

Repro made with npm preact init can be found here:
https://github.com/zakstucke/preact-4194-repro

This repro fails with 10.19.5, works with 10.18.1, and pretty sure it's the root of this issue.

cd my-preact-app && npm i && npm run dev

Going to http://localhost:5173 will give the following error:

The repro is effectively:

import { render } from 'preact';
import { Suspense } from "react";

import { Tab } from "@headlessui/react";

import { TestComp, TestWrapper } from "./lazyBarrier";

export function App() {
	return (
		<Suspense fallback={<p>Loading...</p>}>
			<Tab.Group>
				<Tab>
					<p>Foo</p>
				</Tab>
				<Tab.Panels>
					<Tab.Panel>
						<TestWrapper>
							<TestComp />
						</TestWrapper>
					</Tab.Panel>
				</Tab.Panels>
			</Tab.Group>
		</Suspense>
	);
}

render(<App />, document.getElementById('app'));

Where TestWrapper is a lazy loaded wrapper component (just returning children)
and TestComp is also lazy loaded and just returns <p>Bar</p>

I haven't been able to get @headlessui/react out of the mix, but it should be a start to diagnose!

Thanks

[JoviDeCroock]

Hmm, I think that narrows it down to #4182, after some testing it does not seem to be the aforementioned issue. 4182 is a pretty big release so will look into this when I have some more time.

[JoviDeCroock]

Okay, I had some time today to get to the bottom of this...

The main issue is that in nested fragment-like children we can have a very deep chain of components where the last one eventually resolves to unmounting the children.

This effectively means that _nextDom on all parents is going to be invalid, we however can't course-correct as we've already higher-up in the stack assigned oldDom https://github.com/preactjs/preact/blob/main/src/diff/children.js#L61 which now in-turn can become invalid by the first call to diff https://github.com/preactjs/preact/blob/main/src/diff/children.js#L85

[zakstucke]

@JoviDeCroock I can confirm this actually fixed it! Thanks so much for all your help :)