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

When navigating ForYou → Topic → switch tab → back, the user landed on
the ForYou screen instead of the Topic screen. The root cause was that
toEntries() flattened all tab sub-stacks into a single entries list,
causing ListDetailSceneStrategy to mis-render after cross-tab back.

- Change toEntries() to only return entries from the current tab's
  sub-stack while still decorating all stacks for state preservation
- Add BackHandler for cross-tab back navigation since NavDisplay now
  only sees within-tab entries
- Add unit test for the sub-stack preservation scenario
- Un-ignore navigationBar_multipleBackStackInterests test
- Add back-button variant instrumented test

app/src/androidTest/kotlin/com/google/samples/apps/nowinandroid/ui/NavigationTest.kt

@@ -46,7 +46,6 @@ import dagger.hilt.android.testing.HiltAndroidTest
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.runBlocking
 import org.junit.Before
-import org.junit.Ignore
 import org.junit.Rule
 import org.junit.Test
 import javax.inject.Inject
@@ -255,9 +254,6 @@ class NavigationTest {
         }
     }
 
-    // TODO decide if backStack should preserve previous stacks when navigating back to home tab (ForYou)
-    // https://github.com/android/nowinandroid/issues/1937
-    @Ignore
     @Test
     fun navigationBar_multipleBackStackInterests() {
         composeTestRule.apply {
@@ -283,6 +279,32 @@ class NavigationTest {
         }
     }
 
+    @Test
+    fun navigationBar_backFromTabPreservesSubStack() {
+        composeTestRule.apply {
+            onNodeWithText(interests).performClick()
+
+            // Select a topic
+            val topic = runBlocking {
+                topicsRepository.getTopics().first().sortedBy(Topic::name).last()
+            }
+            onNodeWithTag(LIST_PANE_TEST_TAG).performScrollToNode(hasText(topic.name))
+            onNodeWithText(topic.name).performClick()
+
+            // Verify the topic is shown
+            onNodeWithTag("topic:${topic.id}").assertIsDisplayed()
+
+            // Switch to another tab
+            onNodeWithText(saved).performClick()
+
+            // Press back to return to previous tab
+            Espresso.pressBack()
+
+            // Verify the topic detail is still shown (sub-stack preserved)
+            onNodeWithTag("topic:${topic.id}").assertExists()
+        }
+    }
+
     @Test
     fun navigatingToTopicFromForYou_showsTopicDetails() {
         composeTestRule.apply {

app/src/main/kotlin/com/google/samples/apps/nowinandroid/ui/NiaApp.kt

@@ -16,6 +16,7 @@
 
 package com.google.samples.apps.nowinandroid.ui
 
+import androidx.activity.compose.BackHandler
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.WindowInsets
@@ -264,8 +265,16 @@ internal fun NiaApp(
                         searchEntry(navigator)
                     }
 
+                    val navigationState = appState.navigationState
+                    BackHandler(
+                        enabled = navigationState.currentKey == navigationState.currentTopLevelKey &&
+                            navigationState.currentTopLevelKey != navigationState.startKey,
+                    ) {
+                        navigator.goBack()
+                    }
+
                     NavDisplay(
-                        entries = appState.navigationState.toEntries(entryProvider),
+                        entries = navigationState.toEntries(entryProvider),
                         sceneStrategy = listDetailStrategy,
                         onBack = { navigator.goBack() },
                     )

core/navigation/src/main/kotlin/com/google/samples/apps/nowinandroid/core/navigation/NavigationState.kt

@@ -96,7 +96,5 @@ fun NavigationState.toEntries(
         )
     }
 
-    return topLevelStack
-        .flatMap { decoratedEntries[it] ?: emptyList() }
-        .toMutableStateList()
+    return (decoratedEntries[currentTopLevelKey] ?: emptyList()).toMutableStateList()
 }

core/navigation/src/test/kotlin/com/google/samples/apps/nowinandroid/core/navigation/NavigatorTest.kt

@@ -247,6 +247,32 @@ class NavigatorTest {
         assertThat(navigationState.currentTopLevelKey).isEqualTo(TestFirstTopLevelKey)
     }
 
+    @Test
+    fun testSubStackPreservedAfterTabSwitchAndBack() {
+        // Navigate to sub-page in first tab (ForYou → Topic)
+        navigator.navigate(TestKeyFirst)
+
+        assertThat(navigationState.currentKey).isEqualTo(TestKeyFirst)
+        assertThat(navigationState.currentTopLevelKey).isEqualTo(TestFirstTopLevelKey)
+
+        // Switch to second tab (e.g. Bookmarks)
+        navigator.navigate(TestSecondTopLevelKey)
+
+        assertThat(navigationState.currentKey).isEqualTo(TestSecondTopLevelKey)
+        assertThat(navigationState.currentTopLevelKey).isEqualTo(TestSecondTopLevelKey)
+
+        // Press back from second tab
+        navigator.goBack()
+
+        // Should return to first tab with sub-stack preserved
+        assertThat(navigationState.currentTopLevelKey).isEqualTo(TestFirstTopLevelKey)
+        assertThat(navigationState.currentKey).isEqualTo(TestKeyFirst)
+        assertThat(navigationState.currentSubStack).containsExactly(
+            TestFirstTopLevelKey,
+            TestKeyFirst,
+        ).inOrder()
+    }
+
     @Test
     fun throwOnEmptyBackStack() {
         assertFailsWith<IllegalStateException> {