back() doesn't load route after hard reload on a shallow-pushed URL after `pushState`

[motherthestate]

Describe the bug

I've encountered a weird behavior with shallow routing, don't know if this is a bug or limitation. Let me know :)

Reproduction

After opening a route as a shallow overlay with pushState, reloading on that URL, then
pushing a second, stacked pushState on top, the browser Back button updates page.url
but never swaps the mounted route. The wrong page component stays on screen.

Here a repo: https://github.com/motherthestate/svelte-shallow-repo

Logs

System Info

System:
    OS: macOS 26.5.1
    CPU: (8) arm64 Apple M1
    Memory: 92.88 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 26.2.0 - /Users/maxime.welter/.nvm/versions/node/v26.2.0/bin/node
    Yarn: 1.22.19 - /opt/homebrew/bin/yarn
    npm: 11.13.0 - /Users/maxime.welter/.nvm/versions/node/v26.2.0/bin/npm
    pnpm: 8.15.4 - /opt/homebrew/bin/pnpm
    bun: 1.3.14 - /opt/homebrew/bin/bun
    Deno: 1.40.4 - /Users/maxime.welter/.deno/bin/deno
  Browsers:
    Chrome: 150.0.7871.129
    Firefox: 152.0.6
    Safari: 26.5
  npmPackages:
    @sveltejs/adapter-auto: ^7.0.1 => 7.0.1 
    @sveltejs/kit: ^2.63.0 => 2.70.1 
    @sveltejs/vite-plugin-svelte: ^7.1.2 => 7.2.0 
    svelte: ^5.56.1 => 5.56.7 
    vite: ^8.0.16 => 8.1.5

Severity

annoyance

Additional Information

No response

[motherthestate]

Found #15618 after creating this, likely the same issue.

[Nic-Polumeyv]

Back takes the shallow branch of the popstate handler instead of navigating:

kit/packages/kit/src/runtime/client/client.js

Lines 2886 to 2887 in 08cfba0

	const shallow =
	navigation_index === current_navigation_index && (has_navigated || is_hash_change);

The index survives the reload in history.state, and the post-reload guard relies on has_navigated being false, but

kit/packages/kit/src/runtime/client/client.js

Lines 2532 to 2533 in 08cfba0

	history.pushState(opts, '', resolve_url(url));
	has_navigated = true;

so Back applies url and state from the stale entry without navigating. Unchanged since #11307, reworked in #16389/#16449.