Fix incorrect back navigation to ForYou instead of Topic screen (#1866)

[Amir-yazdanmanesh]

• Fix cross-tab back navigation losing sub-stack state — pressing back after switching tabs now correctly returns to the previously viewed sub-page (e.g. Topic detail)
  instead of the tab root (ForYou)
• Change toEntries() to only pass the current tab's entries to NavDisplay, preventing ListDetailSceneStrategy from mis-rendering after cross-tab transitions
• Add BackHandler for tab-level back navigation since NavDisplay now only sees within-tab entries

Root Cause

toEntries() in NavigationState.kt flattened all tab sub-stacks into a single entries list. When pressing back from another tab, entries transitioned from [ForYouEntry,
TopicEntry, BookmarksEntry] to [ForYouEntry, TopicEntry], but ListDetailSceneStrategy failed to re-render the detail pane correctly because ForYouEntry lacks pane
metadata while TopicEntry has detailPane() metadata.

Test plan

• Unit test added: testSubStackPreservedAfterTabSwitchAndBack in NavigatorTest.kt
• Un-ignored navigationBar_multipleBackStackInterests instrumented test
• Added navigationBar_backFromTabPreservesSubStack instrumented test with system back button
• Manual verification: ForYou → tap topic → switch to Bookmarks → press back → Topic screen should be visible

Fixes #1866

[gemini-code-assist]

Summary of Changes

Hello @Amir-yazdanmanesh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical navigation bug where the application would lose its sub-stack state when navigating back after switching between tabs. The changes ensure that the back button correctly restores the user to their previous sub-page within a tab, rather than resetting to the tab's root. This significantly improves the user experience by maintaining expected navigation behavior across the application's tabbed interface.

Highlights

• Cross-Tab Back Navigation Fix: Resolved an issue where pressing back after switching tabs would incorrectly return to the tab root (ForYou) instead of the previously viewed sub-page (e.g., Topic detail), ensuring sub-stack state is preserved.
• Navigation Entry Filtering: Modified the toEntries() function to pass only the current tab's navigation entries to NavDisplay, which prevents ListDetailSceneStrategy from mis-rendering after cross-tab transitions.
• BackHandler Implementation: Introduced a BackHandler for tab-level back navigation, as NavDisplay now only processes entries within the active tab.

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Changelog

• app/src/androidTest/kotlin/com/google/samples/apps/nowinandroid/ui/NavigationTest.kt
  • Un-ignored navigationBar_multipleBackStackInterests test.
  • Added navigationBar_backFromTabPreservesSubStack instrumented test.
• app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/NiaApp.kt
  • Imported BackHandler composable.
  • Added a BackHandler to enable correct tab-level back navigation.
• core/designsystem/src/main/kotlin/com/google/samples/apps/nowinandroid/core/designsystem/component/scrollbar/AppScrollbars.kt
  • Removed SuppressLint import.
  • Removed @SuppressLint("ComposableModifierFactory") annotation.
• core/navigation/src/main/kotlin/com/google/samples/apps/nowinandroid/core/navigation/NavigationState.kt
  • Modified toEntries function to only provide entries for the current top-level tab.
• core/navigation/src/test/kotlin/com/google/samples/apps/nowinandroid/core/navigation/NavigatorTest.kt
  • Added testSubStackPreservedAfterTabSwitchAndBack unit test.

Activity

• Added a new unit test, testSubStackPreservedAfterTabSwitchAndBack, to verify sub-stack preservation.
• Un-ignored the existing navigationBar_multipleBackStackInterests instrumented test.
• Introduced a new instrumented test, navigationBar_backFromTabPreservesSubStack, to validate system back button behavior.
• Manual verification steps were outlined to confirm the fix for cross-tab back navigation.

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[gemini-code-assist]

Code Review

The pull request successfully addresses the issue where back navigation from a sub-stack would incorrectly return to the app root instead of preserving the previous tab's state. The solution of isolating entries per tab in toEntries() and adding a BackHandler for cross-tab navigation is a solid architectural improvement that also resolves rendering issues with ListDetailSceneStrategy. The added unit and instrumented tests provide good coverage for the fix.