`resize-observer@4.0.0-next.3` causes hydration mismatch

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
68/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
typescript
Domain
frontend

Research direction

Start in packages/resize-observer/src/index.ts around line 201, where createEffect is gated by isServer. Compare server and client hydration behavior using the provided ElementSizeHydrationRepro component. Done means the resize observer no longer causes a hydration mismatch while preserving the intended size behavior; the Solid2 behavior discussion may require separate clarification.

Written by the indexing model from the issue text.

Description

bug
Describe the bug
import { createSignal } from "solid-js";
import { createElementSize } from "@solid-primitives/resize-observer";

export default function ElementSizeHydrationRepro() {
  const [target, setTarget] = createSignal<HTMLDivElement>();
  const size = createElementSize(target);

  return <><div ref={setTarget}>Width: </div>{size.width ?? "waiting"}</>;
}

The cause is that createEffect advances hydration id but it's only created on the client:

https://github.com/solidjs-community/solid-primitives/blob/74ac166a63ccf7203aa4e6b0801f79ff7491aab3/packages/resize-observer/src/index.ts#L201

A workaround would be wrapping the use site in a new owner. A proper fix would be always createEffect regardless of isServer or not.

BTW, I believe that the idiomatic Solid2 way of writing this would be creating a writable signal with ssrSource: "client". This way the returned size can be non-null, but the result must be wrapped in a separate <Loading> boundary or else the component could never render due to the loop render -> get size -> observed el not mounted until rendered -> isPending, abort render. It sounds problematic at first but it actually prevents the render loop of using the size inside the measured element. However, that's a behavior change and needs discussion.

Minimal Reproduction Link

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