fix(Android, Stack v5): preventNativeDismiss survives activity restart

[kkafar]

Description

Stacked on #4578 (header chevron scoped to its own stack).

preventNativeDismiss on Android (Stack v5) silently stops intercepting the system back press after the activity goes through one stop/start cycle (app switch, lock screen, home button). From then on the screen pops as if the flag were off, until its fragment is recreated.

Root cause (verified against androidx.activity 1.8.1 / androidx.fragment 1.8.9): OnBackPressedDispatcher hands a back press to the last-added enabled callback. Every FragmentManager registers its pop callback with the lifecycle-owner overload, which removes it on the owner's ON_STOP and re-appends it at the deque tail on ON_START. Our per-fragment PreventNativeDismissCallback was registered with the plain overload, so its slot never moved. One background cycle was enough for every FragmentManager callback to leapfrog every prevent callback.

Fix: each StackContainer owns a single veto-only OnBackPressedCallback, registered with the same lifecycle owner its FragmentManager uses (the hosting fragment when nested, FragmentActivity's fragment host at the root). LifecycleRegistry re-dispatches ON_START in registration order, so after every restart the deque is rebuilt as [fm, veto] per level. Deeper fragments start after their parents, so a deeper stack with something to pop still beats a shallower veto. FragmentManager keeps executing pops, so predictive back is unaffected. System back now resolves through the same recursive wantsToPreventStackNativeDismiss() query as the header chevron from #4578.

Note: the root owner is obtained through FragmentManager.getHost(), which is @RestrictTo(LIBRARY) (lint-suppressed). There is no public accessor. The activity's own lifecycle cannot be used: it starts after every fragment and would put the root veto behind all nested FragmentManager callbacks.

Closes https://github.com/software-mansion/react-native-screens-labs/issues/1775

Changes

• StackContainer: new System back veto region. Lifecycle-owned veto callback (registered on attach, removed on detach), enabled predicate recomputed eagerly, re-dispatch to the next callback when the state went stale, emitNativeDismissPrevented shared with the chevron path, ancestor invalidation walk.
• FragmentManagerHelper: findFragmentManagerWithOwnerForView returns the manager together with its lifecycle owner and dispatcher. findFragmentManagerForView is now non-null (it never returned null).
• StackScreen: preventNativeDismiss changes invalidate the owning containers instead of notifying a single observer.
• Removed: PreventNativeDismissCallback, PreventNativeDismissChangeObserver, top-fragment bookkeeping in StackScreenFragment, updateTopFragment + OnCommitCallbackOp.
• TabsContainer: null-safe selectedTabOrNull for the veto query (reachable after the selected tab was removed).

Test plan

Repro / regression: apps/src/tests/issue-tests/TestNestedStackPreventSystemBack.tsx (launch directly via apps/App.tsx): push NestedStack → system back intercepted (toast) → background + foreground → system back still intercepted. Push NestedA → system back pops only NestedA, before and after backgrounding.

Regression scenarios: apps/src/tests/single-feature-tests/stack-v5/test-stack-prevent-native-dismiss-single-stack/scenario.md and .../test-stack-prevent-native-dismiss-nested-stack/scenario.md (all steps, plus a background cycle after an interception), root-exit block with a single preventing screen, predictive back gesture where nothing vetoes, tabs smoke (test-stack-tabs-stack-in-tabs-base-navigation).

Detox: FabricExample/e2e/single-feature-tests/stack-v5/test-stack-prevent-native-dismiss-{single,nested}-stack.e2e.ts.

Status: static checks green (yarn lint-android, assembleDebug). On-device verification and the scenario/doc-comment updates are pending, hence draft.

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types.
• Ensured that CI passes

🤖 Generated with Claude Code

https://claude.ai/code/session_01AqrHqNcfhAe6Epx5XGhMWB

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