Broken tutorial due to `data` being immutable in Svelte 5

[henrykrinkle01]

Today somebody in the Discord server asked why the UI doesn't update with the official solution:

https://svelte.dev/tutorial/kit/post-handlers

The solution worked in Svelte 4 because the data props was mutable. In Svelte 5 it's no longer the case. This issue has been brought up several times:
#12999
#14536

I assume that the tutorial is migrated from Svelte 4 using the migration tool which just changes export let data to let { data = $bindable() } = $props() which currently does nothing.

I offered this answer:

    let { data: _data } = $props();
    let data = $state(_data);
    $effect(() => {
        data = _data;
    });

However, is this pattern really what should be put into the official tutorial to teach new people about Svelte/Kit?

To my understanding, there are 3 possible solutions:

• Do nothing, just use $effect, which I only consider as a workaround.
• Make data = $bindable() work, which should make data and page.data mutable for optimistic UI updates. Best solution IMO. It worked in Svelte 4 so why can't it work in Svelte 5?
• Something else like $state.link.

So what should be done about this tutorial in particular and the underlying problem in general?