Bug Reproduction: Svelte Async Reactivity Issue

Description

This repository demonstrates a reactivity failure in Svelte 5 when the experimental async compiler is enabled. During a specific navigation sequence, the DOM fails to update to the new route's content, leaving the previous page's UI visible despite the URL changing.

Environment Details

• Svelte: 5.46.0
• SvelteKit: 2.49.2
• Vite: 7.3.0
• Compiler Option: experimental: { async: true }
• Package Manager: pnpm

Reproduction Steps

1. Install & Start:

pnpm install
pnpm run dev --open /first

2. Navigation Path:

• Step 1: Start at /first (used as a clean entry point).
• Step 2: Click the link to navigate to "Second" (/second).
• Step 3: From the /second page, click the link to "Third" (/second/some-id).

Observed Behavior

Expected Behavior

After Step 3, the UI should render the content of src/routes/second/[id]/+page.svelte.

Expected Text: "Third Page - Success" (inside the red box).

Actual Behavior

The UI remains stuck on the state of the previous route.

Actual Text: "Second Page" persists in the red box.

Project Structure & Findings

The bug appears to be sensitive to the routing hierarchy and the presence (or absence) of layout files.

.
├── svelte.config.js           # Includes experimental: { async: true }
└── src/
    └── routes/
        ├── +layout.svelte     # Root layout
        ├── first/
        │   ├── +layout.svelte
        │   └── +page.svelte   # Start here
        └── second/
            ├── [id]/
            │   └── +page.svelte  # "Third" page (Fails to render)
            ├── +page.svelte      # "Second" page
            └── +layout.svelte.removed  # Observation: Renaming this to +layout.svelte
                                        # causes the bug to disappear.

Key Observations

• Sequence is Important: The failure only occurs when navigating sequentially from /second to /second/[id]. Direct navigation to /second/[id] (or navigating there from /first) does not seem to trigger the hang.
• Compiler Dependency: The issue is strictly tied to the experimental.async: true flag. If this flag is disabled, the navigation works as expected.
• Layout Influence: If a +layout.svelte file exists in the second/ directory, the bug disappears. While Svelte does not require a layout here, adding one appears to change how the async compiler manages the transition between the parent and child route, preventing the UI from getting stuck