`$derived` not updated on first read if source is async store

[mcous]

Describe the bug

If a $derived rune derives its value from a asynchronous store (i.e. one that uses StartStopNotifier), its value is not updated on first read.

We ran into this extremely fun issue when upgrading a stores-based state structure to runes. One of the sources to a $derived value is a @tanstack/svelte-query store that updates asynchronously. The $derived value is not accessed directly in the template, but is accessed in a callback triggered on user click. When the callback is triggered, the $derived value is stale and does not recalculate, even though every source value before the $derived is up to date.

In our case, this issue only manifested in the prod build - not the dev build - but I was able to boil it down to a reproduction that occurs in dev, linked below. In our actual code, the rune is part of an object attached to a context, which for some reason is enough to get this working on our dev server.

Reproduction

https://svelte.dev/playground/31db259839d54528a3323b9987bce475?version=5.23.0

System Info

System:
  OS: macOS 14.3.1
  CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  Memory: 516.24 MB / 32.00 GB
  Shell: 5.9 - /usr/local/bin/zsh
Binaries:
  Node: 22.14.0 - ~/.nvs/default/bin/node
  npm: 10.9.2 - ~/.nvs/default/bin/npm
  pnpm: 9.15.5 - ~/.nvs/default/bin/pnpm
Browsers:
  Chrome: 123.0.6312.86
  Safari: 17.3.1
npmPackages:
  svelte: ^5.20.2 => 5.23.0

Severity

blocking an upgrade

[mcous]

Continuing to poke at this, and it looks like my reproduction is too minimal and is just showing expected behavior. Apologies for the noise; I will re-open a new issue with a better repro when I have it