Describe the bug

$derived returns wrong value in $effect.pre and attachment cleanup during unmount.

When a component unmount is triggered by state written inside an $effect (a common host→widget sync pattern), reading a $derived during teardown returns the post-change value instead of the pre-change value. This breaks the teardown contract established by #15469 and is the root cause of #15601.

In practice, this means $derived(data.label) inside a teardown throws TypeError: Cannot read properties of undefined when the {#if} guard that protects the render path doesn't protect teardown reads.

The behavior is also inconsistent across teardown types — the same $derived read returns different values depending on where the cleanup runs:

Without the $effect wrapper (setting state directly), all four correctly see the pre-change value.

Conditions required:

1. The state change that triggers the unmount happens inside an $effect
2. The $derived values are read in the template (so they have a cached pre-change value)

Root cause (verified on instrumented 5.51.2): when the write happens inside an effect, the dependent {#if} block is processed through the eager path in flush_queued_effects, which calls old_values.clear() before the destroy teardowns run. Suppressing that clear() restores correct pre-change values for all four positions.

Related: #15601 (this is its root cause), #15469, #14025.

Reproduction

https://svelte.dev/playground/3170f9b03bad47228551d7435a8be42c?version=5.56.2

System Info

System:
  OS: macOS 26.5.1
  CPU: (10) arm64 Apple M4
  Memory: 451.00 MB / 16.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 24.15.0 - /Users/kiv/.nvm/versions/node/v24.15.0/bin/node
  npm: 11.12.1 - /Users/kiv/.nvm/versions/node/v24.15.0/bin/npm
  pnpm: 10.33.3 - /Users/kiv/Library/pnpm/pnpm
Browsers:
  Chrome: 148.0.7778.179
  Firefox: 149.0
  Safari: 26.5
npmPackages:
  rollup: ^4.57.1 => 4.57.1
  svelte: ^5.56.2 => 5.56.2

Severity

annoyance