fix(iOS, Tabs): resolve content scroll view for tabBarMinimizeBehavior in real-world RN hierarchies

[sgup]

Note: The code / PR is made by Claude Fable, however I've manually tested and confirmed with my app on simulator and device.

Description

On iOS 26, tabBarMinimizeBehavior: 'onScrollDown' never engages when a tab's content is a nested (native stack) navigator and/or the scrolling content is a virtualized list (FlashList, LegendList) or any UIScrollView behind styled wrapper views — i.e. the common shape of a production app. The tab bar never minimizes and a bottom accessory never reaches its inline placement.

UIKit resolves the scroll view that drives minimize by asking the selected tab's view controller for its bottom-edge content scroll view (contentScrollView(for:) / automatic detection). UIKit's automatic detection cannot find a UIScrollView nested inside a React Native screen tree. The collapsable={false} workaround from #3954 only addresses the trivial single-styled-wrapper case — it cannot help when the first-descendant chain runs through a nested stack host or a virtualized list's wrappers.

Closes #4145.

Changes

• RNSScrollViewFinder: new findScrollViewBreadthFirstFrom: — a bounded breadth-first search (2000-view visit cap) that skips horizontal-only scrollers (carousels), so behaviors driven by the result track the vertical list. The existing first-descendant-chain walk is exactly what fails on real screens.
• RNSTabsScreenViewController + RNSScreen: implement contentScrollViewForEdge: (UIKit's documented override point), covering whichever controller UIKit queries (the tab root, or the visible nested-stack screen). UIKit's own resolution is tried first; the BFS is only a fallback, and only for the bottom edge so top-edge (navigation bar) resolution is unaffected.

Open questions for maintainers:

• The RNSScreen override applies to all stack screens, not only tab-hosted ones. We saw no regressions (bottom edge only), but it could be gated to tab-hosted screens or behind a prop if preferred.
• An alternative shape would be extending the existing RNSContentScrollViewProviding delegation so the BFS lives behind the provider protocol.

Test plan

Tested in a production Expo SDK 56 app (RN 0.85, new arch) on a physical iOS 26 device, with these changes applied to 4.25.2 via package patching (this area is unchanged between 4.25.2 and main):

• Tabs each hosting a nested native stack whose screens render LegendList content behind styled wrappers/headers: scrolling down now minimizes the tab bar and moves the bottom accessory to its inline placement on every tab; scrolling up restores it. Previously minimize never engaged anywhere.
• Screens with a horizontal carousel near the top: minimize still tracks the vertical list (carousel skipped by the finder).
• Navigation-bar / top-edge behaviors unaffected (override restricted to the bottom edge).

Reproduction shape:

NativeTabs (tabBarMinimizeBehavior: onScrollDown)
└─ Tab
   └─ native stack navigator
      └─ screen
         └─ styled wrappers / headers
            └─ FlashList / LegendList / ScrollView

Same class of issue reported against react-native-bottom-tabs (callstack/react-native-bottom-tabs#496), where swizzling contentScrollView(for:) is the community fix; this PR uses the documented override point instead.

Checklist

• Included code example that can be used to test this change.
• For API changes, updated relevant public types. (No public API changes; new method is internal.)
• Ensured that CI passes