fix(iOS): unregister tabs accessory observer from observed wrapper

[safaiyeh]

Description

Fixes a possible iOS 26 bottom accessory crash during invalidation.

The helper registers KVO for center on self.nativeWrapperView, but invalidation removed the observer from _bottomAccessoryView.superview.superview at teardown time. UIKit can detach or swap the bottom accessory wrapper before invalidation runs, so the teardown path may attempt to remove the observer from a different wrapper view and raise an NSRangeException because that object was never observed.

I do not see an existing issue for this exact stack, but we observed it in production on react-native-screens 4.23.0, and the same registration/removal pattern is present on current main.

Changes

• Stores the exact native wrapper view that was registered for KVO.
• Makes registerForAccessoryFrameChanges idempotent for the same wrapper and unregisters the previous wrapper if the wrapper changes.
• Removes the observer from the stored observed wrapper during invalidation instead of resolving the current superview chain again.
• Uses a dedicated KVO context and forwards unrelated observations to super.

Test plan

• yarn install --immutable
• yarn prepare
• yarn check-types
• xcrun clang-format -i ios/tabs/bottom-accessory/RNSTabsBottomAccessoryHelper.mm
• git diff --check

Existing examples that exercise bottom accessory behavior:

• apps/src/tests/issue-tests/Test3288.tsx
• apps/src/tests/single-feature-tests/tabs/bottom-accessory-layout.tsx

I did not add a JS unit test for this because the failure is caused by UIKit/KVO object identity during native view invalidation. A meaningful automated regression test would need a native iOS XCTest harness or an iOS integration/e2e test that drives bottom accessory mount, detach, and invalidation lifecycle. This repository does not appear to have an iOS native unit-test target for library internals today.

I attempted yarn test:unit after initializing the react-navigation submodule, but the root Jest command also collects the submodule's own test suites and fails on unrelated submodule Jest setup / React version issues before exercising this native change.

Checklist

• Included code example that can be used to test this change. Existing bottom accessory examples are listed above; no new JS example was added for this native KVO lifecycle fix.
• For visual changes, included screenshots / GIFs / recordings documenting the change. N/A, no visual changes.
• For API changes, updated relevant public types. N/A, no API changes.
• Ensured that CI passes

[Copilot]

Pull request overview

Fixes a potential iOS 26 crash during bottom accessory invalidation by ensuring KVO is unregistered from the exact wrapper view instance that was originally observed (rather than re-deriving the wrapper via the current superview chain at teardown time).

Changes:

• Track and store the specific native wrapper view used for KVO registration, and unregister from that stored instance during invalidation.
• Make frame-change KVO registration idempotent for the same wrapper and safely re-register if the wrapper view changes.
• Use a dedicated KVO context and forward unrelated observations to super.

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[kligarski]

Hi, @safaiyeh. Thank you for the PR, it looks good.

I'll try to push a change with removal of the redundant boolean (see comment below) and merge the changes after CI passes.

[kligarski]

It seems to me that this boolean is redundant. If _observedNativeWrapperView is nil, we know that we don't observe anything. It also introduces possibility of inconsistent state (e.g. _isObservingNativeWrapperView = YES and _observedNativeWrapperView = nil) so I don't think we should keep it.

[safaiyeh]

great glad I could help!