Bug in hash-based routing

[sateeshsai]

Describe the bug

When router type is set to hash, the browser navigation works fine in dev mode (pnpm run dev). But the navigation is inconsistent/incorrect in build.

For example, from route #/, if you navigate to /#route1 and go back to #/ by clicking on the back button of the browser, you can't go forward to #/route1 anymore using browser forward button (disabled).

Reproduction

• pnpx sv create
• Sveltekit minimal
• Set routing type to hash
• Bundle strategy to inline and adapter set to @sveltejs/adapter-static
• create a route route1
• pnpm run build pnpm run preview
• From #/ go to #/route1 and come back to #/ using browser back button. You will see that you can't go forward to #/route1 using browser forward button

Minimal reproduction repo:
https://github.com/sateeshsai/hash-routing-issue

Logs

None

System Info

System:
    OS: macOS 15.2
    CPU: (12) arm64 Apple M2 Max
    Memory: 537.56 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.12.2 - /usr/local/bin/node
    npm: 10.5.2 - /usr/local/bin/npm
    pnpm: 9.0.5 - /usr/local/bin/pnpm
  Browsers:
    Chrome: 133.0.6943.54
    Edge: 133.0.3065.59
    Safari: 18.2
  npmPackages:
    @sveltejs/adapter-auto: ^4.0.0 => 4.0.0 
    @sveltejs/adapter-static: ^3.0.8 => 3.0.8 
    @sveltejs/kit: ^2.16.0 => 2.17.1 
    @sveltejs/vite-plugin-svelte: ^5.0.0 => 5.0.3 
    svelte: ^5.0.0 => 5.20.0 
    vite: ^6.0.0 => 6.1.0

Severity

blocking all usage of SvelteKit

Additional Information

No response

[stephenlrandall]

Some additional notes:

• Without bundleStrategy === 'inline': The browser forward/backward buttons seem to work on all browsers except Safari, where the navigation stack duplicates each entry. Navigate to "Route 1" in the reproduction and then back to "Home". The browser's back button then takes you to "Route 1" -> "Route 1" (again) -> "Home" -> "Home" (again). (I wonder if this is related to 404 when loading inline app with hash in URL (Safari only) #13318.) This behavior does not happen in Chrome.
  • This occurs for both dev and preview.
• With bundleStrategy === 'inline': The behavior described in the issue occurs both in preview and when opening the built index.html in a browser.

[stephenlrandall]

In the reproduction, if you instead replace the routes with a single [slug] route, the history gets messed up even during dev. Tossing some logging into Kit's navigate function in runtime/client/client.js yields:

The consecutive /route1 logs are a product of a single back button click, so the issue seems to be multiple navigate calls, the second of which has the right combination of arguments to trigger clear_onward_history.