[Async] Nested layouts + client-side nav results in page store not being populated quickly

Describe the bug

When compilerOptions.experimental.async: true is enabled, reactive function calls in templates receive stale/incomplete $page.data during client-side navigation, while reactive statements see complete data in the same render cycle.

Expected vs Actual

Expected (async: false or page reload):

• Item count: 3 (correct)
• Reactive function calls see complete data

Actual (async: true + client-side nav):

• Item count: 0 (incorrect)
• Same data exists in reactive statements and template conditionals
• Only function calls within template get stale data

Key Details

• Server-side render (page reload): Works correctly
• Client-side navigation: Bug appears
• Bug pattern: Reactive inconsistency within single render cycle
• Data structure: 4 nested layouts + page data via await parent()

The same $page.data store returns different values to different reactive contexts during client-side navigation when async mode is enabled.

Reproduction

https://github.com/PuruVJ/repro-async-bug-nested-layouts

1. Clone this reproduction app
2. Run npm install && npm run dev
3. Go to http://localhost:5173
4. Click "Go to nested page (where the bug occurs)" link
5. Observe the "Item count" value

Expected vs Actual Behavior

With async: true (current config):

Client-side navigation (clicking link):

• Item count: 0 ❌ (incorrect)
• Has apiData: true
• Full data shows 3 items in array

Page reload (F5 or direct URL):

• Item count: 3 ✅ (correct)
• Same complete data

With async: false (comparison):

1. Change svelte.config.js: set async: false
2. Restart dev server
3. Test both navigation methods:

Client-side navigation (clicking link):

• Item count: 3 ✅ (correct)

Page reload (F5 or direct URL):

• Item count: 3 ✅ (correct)

Logs

System Info

System:
    OS: macOS 26.0.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 5.25 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.1.0 - /opt/homebrew/bin/node
    npm: 11.3.0 - /opt/homebrew/bin/npm
    pnpm: 10.19.0 - /opt/homebrew/bin/pnpm
  Browsers:
    Chrome: 141.0.7390.109
    Safari: 26.0.1
  npmPackages:
    @sveltejs/adapter-auto: ^7.0.0 => 7.0.0 
    @sveltejs/kit: ^2.47.3 => 2.47.3 
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1 
    svelte: ^5.41.2 => 5.41.2 
    vite: ^7.1.12 => 7.1.12

Severity

blocking an upgrade

Additional Information

No response

Start with the linked reproduction app, install its dependencies, and run npm install && npm run dev. Compare client-side navigation with a page reload while async is enabled, then compare with async disabled; done means the item count and page data are consistent in both navigation paths.