fix(router): do not inherit shallow flag from history state on browser back/forward (popstate)

[sleitor]

What

Fixes #93844

When router.replace() is called with shallow: true, Next.js stores the shallow flag inside window.history.state for that history entry. This is correct for the immediate navigation — it suppresses the getServerSideProps call as intended.

However, the flag is never cleared, so when the user later navigates back (or forward) via the browser, the onPopState handler reads options.shallow from the stored history state and re-applies it, incorrectly skipping the data fetch again.

How

In the onPopState handler, force shallow: false regardless of what is stored in the history state:

// Before
shallow: options.shallow && this._shallow,

// After — browser back/forward must never be shallow
shallow: false,

Browser back/forward navigations (popstate events) should always trigger a fresh getServerSideProps call. The _shallow flag in the history state was only meant to suppress the fetch at the time of the original replace/push call.

Testing

Added a regression test to the existing client-shallow-routing E2E suite that:

1. Visits a page
2. Calls router.replace() with shallow: true (no data fetch expected)
3. Navigates to another page
4. Presses browser back — verifies that getServerSideProps is called (new random value)

Related

• getServerSideProps not called if book-ended by shallow routes.  #34365 — original report of similar behavior
• Navigating Between Dynamic Routes with Browser Back Button Breaks After Shallow Routing #16028 — related history state / shallow routing issue