Page props are re-set whenever session store is updated

[ponderingexistence]

Describe the bug

In the docs, in the section about the load function, it's stated that:

...if url, session or stuff are used in the function, they will be re-run whenever their value changes... (here)

But this doesn't seem to be the case. I have the following page in my SvelteKit app:
index.svelte:

<script lang="ts" context="module">
    import type { Load } from '@sveltejs/kit';

    export const load: Load = () => {
        console.warn('index load function');
        return {};
    }
</script>

<script lang="ts">
    import { session } from '$app/stores';

    function changeSession() {
        $session.someField = Math.random();
    }
</script>

<pre>{JSON.stringify($session, null, 4)}</pre>

<button on:click={changeSession}>
    Change $session
</button>

If you click on the "Change $session" button, you'll see the message "index load function" appearing in the console, which obviously means the load function is re-run, even though session isn't used in the load function. This is inconsistent with the behavior described in the docs.

The strange thing, however, is that from the second time onwards, when the value of the '$session' store changes, the load function is no longer re-run. This is confusing.

The more severe bug, however, which seems to be related to this one, is that every time the $session store changes, the page's props are apparently re-set, which means reactivity is re-triggered throughout the entire page, potentially causing massive problems.

Consider the following:

<script lang="ts" context="module">
    import type { Load } from '@sveltejs/kit';

    export const load: Load = () => {
        console.warn('index load function');
        return {
            props: {
                userBasicInfo: {
                    id: 2,
                }
            }
        };
    }
</script>

<script lang="ts">
    import { browser } from '$app/env';
    import { session } from '$app/stores';

    export let userBasicInfo;

    function changeSession() {
        $session.someField = Math.random();
    }

    $: console.log('userBasicInfo changed:', userBasicInfo);
</script>

<pre>{JSON.stringify($session, null, 4)}</pre>

<button on:click={changeSession}>
    Change Ssession
</button>

<hr />

{#if browser}
    {#await fetch(`https://reqres.in/api/users/${userBasicInfo.id}`).then(r => r.json())}
        Loading...
    {:then result}
        <pre>{JSON.stringify(result, null, 4)}</pre>
    {/await}
{/if}

In this setup, whenever you click on the "Change $session" button — which is to say whenever you update the $session store — the fetch request in the markup, which uses the userBaicInfo prop, is re-triggered, even though it obviously shouldn't. And you can see the userBasicInfo changed: ... message in the console which proves that it's because of the userBasicInfo prop getting re-set.

Reproduction

You can create an empty SvelteKit skeleton project and then copy-paste the two code snippets above into the index.svelte file, and then npm run dev => open your browser and see the behavior for yourself.

Logs

No response

System Info

System:
    OS: Windows 10 10.0.19042
    CPU: (4) x64 Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
    Memory: 1.09 GB / 7.89 GB
  Binaries:
    Node: 14.15.0 - C:\Program Files\nodejs\node.EXE     
    Yarn: 1.22.10 - ~\AppData\Roaming\npm\yarn.CMD       
    npm: 6.14.8 - C:\Program Files\nodejs\npm.CMD        
  Browsers:
    Edge: Spartan (44.19041.1266.0), Chromium (97.0.1072.76)
    Internet Explorer: 11.0.19041.1202
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.17
    @sveltejs/kit: next => 1.0.0-next.260
    svelte: ^3.44.0 => 3.46.4

Severity

blocking all usage of SvelteKit

[babichjacob]

Duplicate of #3732 -- don't do that

[ponderingexistence]

@babichjacob Well, I didn't get a response in that issue even though pretty much all of the issues that were created after mine did, I thought maybe there's something wrong.

Should I just wait?

[rmunn]

@Aaron2321d - Yes, wait for someone to have time to look at your issue. There are a limited number of Svelte maintainers and volunteers. If you want attention to an issue, mentioning it on the #svelte or #svelte-kit channel (as appropriate) on the Discord would be appropriate, as long as you don't do so too often. No more than once a week would be my recommendation.

That said, you don't need to do so right now, as I'm taking a look at #3732 now and I'll post my findings when I'm done.