A derived value with stores loses reactivity

[hironytic]

Describe the bug

I found that a derived value loses reactivity after upgrading our project to use svelte 5.19.5.
See a minimal sample code in the playground below. Our actural codes are more complex.

Reproduction

https://svelte.dev/playground/990ac378ee8a40e9a168c1856c76de88?version=5.19.7 (Reproducible)

https://svelte.dev/playground/990ac378ee8a40e9a168c1856c76de88?version=5.19.5 (Reproducible)

https://svelte.dev/playground/990ac378ee8a40e9a168c1856c76de88?version=5.19.4 (Works as expected)

When the "Change Store" button is clicked, the value of the store is changed from "previous" to "new". But the text does not change.
If you comment out the line 8, it works fine. So toStore() is related to it, possibly.

Logs

System Info

System:
    OS: macOS 14.7.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 479.19 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.17.0 - ~/.volta/tools/image/node/20.17.0/bin/node
    Yarn: 1.22.19 - ~/.volta/tools/image/yarn/1.22.19/bin/yarn
    npm: 10.9.0 - ~/.volta/tools/image/npm/10.9.0/bin/npm
    pnpm: 9.10.0 - ~/.volta/bin/pnpm
    bun: 1.1.9 - ~/.bun/bin/bun
  Browsers:
    Chrome: 132.0.6834.160
    Safari: 18.1.1
  npmPackages:
    svelte: latest => 5.19.7

Severity

blocking an upgrade

[dummdidumm]

@trueadm this seems to be another regression stemming from #15137 - though it may be that #15137 isn't the direct culprit but merely uncovered a different issue.

The problem is that inside fromStore we're using $effect.tracking() to determine whether or not to create a subscription. When the $derived is read inside the instance script, $effect.tracking() is false, so the store value is only retrieved without subscribing to it. But that means that there's now no dependency within $derived on the store, so it will never notice the updates.