Only pop the pre-redirect controller when a redirect crosses contexts

[svara]

Summary

Redirects were popping one view controller too many. This narrows the pop so it only fires when a redirect crosses the default ↔ modal navigation boundary — fixing the missing / incorrect back button in #190 while preserving the modal-redirect cleanup from #112.

The bug (#190)

Since the 1.3.0 beta, following an internal redirect could leave the navigation stack wrong:

• Redirect on/near the first screen → the tab's root screen is lost and there's no back button.
• Redirect after navigating → a back button appears but returns to the wrong screen (one too far back).

Works correctly in 1.2.x.

Root cause

session(_:didProposeVisit:) popped the top controller on every redirect (introduced in #160, generalizing the modal fix from #154 / #112):

if proposal.isRedirect {
    let animatePop = session === modalSession && proposal.context == .default
    pop(animated: animatePop)
}
route(proposal)

But Turbo already re-proposes a followed redirect with an action: "replace" — it also does history.replaceState on the web side, so the redirect never becomes a history entry. Verified in the bundled turbo.min.js (Visit#followRedirect):

followRedirect(){ … this.response?.redirected &&
  this.adapter.visitProposedToLocation(this.redirectedToLocation,
    { action: "replace", response: this.response, … }) }

So route(proposal) on its own already replaces the pre-redirect controller in place (pushOrReplace → replaceLastViewController). The extra unconditional pop therefore removes a second controller — the screen underneath — which is exactly the lost / wrong back button in #190.

The pop is only actually required when the redirect changes navigation context (default ↔ modal): there the pre-redirect controller was pushed onto one stack while route targets the other, leaving it orphaned (the original #112 problem).

The fix

Pop only when the redirect crosses the context boundary:

if proposal.isRedirect {
    let sessionIsModal = session === modalSession
    let proposalIsModal = proposal.context == .modal
    if sessionIsModal != proposalIsModal {
        // Animate only when receding from the modal session to the default context.
        let animatePop = sessionIsModal && proposal.context == .default
        pop(animated: animatePop)
    }
}
route(proposal)

• Same context (main → main, modal → modal): no pop; the replace action rewrites the pre-redirect controller in place. When the redirect targets the previous screen, pushOrReplace's visitingPreviousPage branch performs the animated back-pop — restoring the transition that went missing in the beta.
• Cross context (default → modal, modal → default): pop the orphaned pre-redirect controller before routing, preserving the Redirect to Modal Shows Different Screens Between iOS and Android #112 fix.

Behavior

redirect	before (current beta)	after
main → main (after a screen)	loses the underlying screen ❌	replaces in place ✅
main → main (at root)	ok	ok ✅
main → modal (#112)	ok	ok ✅
modal → modal	loses the underlying screen ❌	replaces in place ✅
modal → main	dismiss + route	dismiss + route ✅

Relationship to #192

#192 also narrows the pop, but keys it off the session (session === modalSession). A default → modal redirect is re-proposed by the main session (which performed the request), so #192 skips the pop there and re-introduces #112 — the pre-redirect controller is left orphaned beneath the presented modal. It also still double-pops modal → modal. Keying off the context transition instead handles all four quadrants.

Tests

Adds redirect coverage across the full default / modal matrix to NavigationHierarchyControllerTests, driven through the real SessionDelegate entry point (session(_:didProposeVisit:)) rather than route(_:).

[joemasilotti]

Approved assuming this fixes #192!