fix: don't set derived values during time traveling

[paoloricciuti]

Just a failing test because I tried to fix this without much success. This also causes SvelteKit to fail when preloading happens (I though #17147 would've fixed it, but the problem is with blocks that use deriveds).

The issue is that we update derived.v and increase the version even if the state change happens in a fork. However, this means that if in a later state update (be it in another fork or just normal state update) to determine if the block effect is dirty we try to update the derived and then check the wv of it. But since we already updated the value AND the version in the fork the block effect is not executed.

IMHO we should reserve the same treatment we reserve to sources for deriveds and update the value in the batch and not in the derived itself but when I tried to do that a bunch of tests failed.

EDIT: oh my god I think I actually figured it out! I just pushed a commit to fix the issue, I'm just not sure if it's the right solution.

Basically, if batch_values is defined we don't set derived.v, we just increase the version. Now what this means is that if we are changing the derived AND we are not in a tracking context we kinda wasted one computation because the next time around the derived will be re-computed (during the get) but if this time we are in a tracking context batch_values will be updated and the get will read that (the rest of the logic applies as usual with batch_values so when we commit the value is the updated one).

That said, there might be an issue when accessing a derived in a non tracking context when batch_values is defined, but I wasn't able to think of a situation for that. Can you?

Fixes sveltejs/kit#14827
Fixes #17190

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Playground

pnpm add https://pkg.pr.new/svelte@17163

[zqianem]

This looks like it may have the same cause as #17111.

[Rich-Harris]

nice!

[Rich-Harris]

ah crap, forgot a changeset and have to run out now. will fix later if no-one else does

[paoloricciuti]

Here we go #17200