fix(navigation): prevent duplicate Sheet key crash in single-route navigateTo

The single-route navigateTo(route) extension called navigate() directly
without checking whether a sheet was already open. This allowed a second
Sheet(TokenDiscovery) to land on the backstack during a dismiss animation,
crashing SaveableStateProvider with 'Key was used multiple times'.

- Single-route navigateTo now defers via pendingSheetDismiss when a sheet
  is already present, matching the multi-route overload behavior
- Add Sheet dedup guard to the ClearAll navigate branch
- Add async duplicate Sheet sanitization in AppNavHost as a safety net
- Add 4 test cases covering the single-route dismiss-then-replace flow

Author: bmc08gt

apps/flipcash/core/src/main/kotlin/com/flipcash/app/core/extensions/CodeNavigator.kt

@@ -16,7 +16,19 @@ fun CodeNavigator.navigateTo(route: NavKey, options: NavOptions = NavOptions())
     } else {
         route
     }
-    navigate(destination, options)
+    val needsSheet = destination is AppRoute.Main.Sheet
+    val hasSheet = backStack.any { it is AppRoute.Main.Sheet }
+
+    if (hasSheet && needsSheet) {
+        pendingSheetDismiss = {
+            Snapshot.withMutableSnapshot {
+                sheetGeneration++
+                navigate(destination, options)
+            }
+        }
+    } else {
+        navigate(destination, options)
+    }
 }
 
 /**

apps/flipcash/shared/router/src/test/kotlin/com/flipcash/app/router/internal/NavigateToTest.kt

@@ -277,4 +277,68 @@ class NavigateToTest {
     }
 
     // endregion
+
+    // region Single-route navigateTo dismiss-then-replace
+
+    @Test
+    fun `single-route navigateTo with existing sheet sets pendingSheetDismiss`() {
+        val navigator = createNavigator(
+            AppRoute.Main.Scanner,
+            AppRoute.Main.Sheet(AppRoute.Sheets.Wallet),
+        )
+
+        navigator.navigateTo(AppRoute.Sheets.TokenDiscovery, options = quietOptions)
+
+        assertNotNull(navigator.pendingSheetDismiss)
+        // Backstack unchanged until the callback fires
+        assertEquals(2, navigator.backStack.size)
+    }
+
+    @Test
+    fun `single-route navigateTo callback navigates to new sheet after dismiss`() {
+        val navigator = createNavigator(
+            AppRoute.Main.Scanner,
+            AppRoute.Main.Sheet(AppRoute.Sheets.Wallet),
+        )
+
+        navigator.navigateTo(AppRoute.Sheets.TokenDiscovery, options = quietOptions)
+
+        // Simulate dismiss: remove old sheet entry, then callback fires
+        navigator.backStack.removeAt(navigator.backStack.lastIndex)
+        navigator.pendingSheetDismiss!!.invoke()
+
+        val last = navigator.backStack.last()
+        assertIs<AppRoute.Main.Sheet>(last)
+        assertEquals(AppRoute.Sheets.TokenDiscovery, last.initialRoute)
+    }
+
+    @Test
+    fun `single-route navigateTo increments sheetGeneration on dismiss-replace`() {
+        val navigator = createNavigator(
+            AppRoute.Main.Scanner,
+            AppRoute.Main.Sheet(AppRoute.Sheets.Wallet),
+        )
+        val initialGeneration = navigator.sheetGeneration
+
+        navigator.navigateTo(AppRoute.Sheets.TokenDiscovery, options = quietOptions)
+
+        // Simulate dismiss
+        navigator.backStack.removeAt(navigator.backStack.lastIndex)
+        navigator.pendingSheetDismiss!!.invoke()
+
+        assertEquals(initialGeneration + 1, navigator.sheetGeneration)
+    }
+
+    @Test
+    fun `single-route navigateTo without existing sheet navigates directly`() {
+        val navigator = createNavigator(AppRoute.Main.Scanner)
+
+        navigator.navigateTo(AppRoute.Sheets.TokenDiscovery, options = quietOptions)
+
+        assertNull(navigator.pendingSheetDismiss)
+        assertEquals(2, navigator.backStack.size)
+        assertIs<AppRoute.Main.Sheet>(navigator.backStack.last())
+    }
+
+    // endregion
 }

ui/navigation/src/main/kotlin/com/getcode/navigation/AppNavHost.kt

@@ -17,6 +17,8 @@ import androidx.navigation3.scene.OverlayScene
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.snapshotFlow
+import androidx.compose.runtime.snapshots.Snapshot
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.luminance
 import androidx.compose.ui.graphics.toArgb
@@ -35,6 +37,7 @@ import com.getcode.navigation.decorators.rememberNavResultScopeEntryDecorator
 import com.getcode.navigation.results.NavResultStateRegistry
 import com.getcode.navigation.results.rememberNavResultStateRegistry
 import com.getcode.theme.CodeTheme
+import timber.log.Timber
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -60,6 +63,29 @@ fun AppNavHost(
 ) {
     ChangeSystemBarsTheme(CodeTheme.colors.background.luminance() < 0.5f)
 
+    // Safety net: async duplicate Sheet sanitization.
+    // Cannot prevent a same-frame crash, but cleans up residual duplicates
+    // from unforeseen race conditions before the next frame renders.
+    LaunchedEffect(navigator.backStack) {
+        snapshotFlow { navigator.backStack.toList() }
+            .collect { stack ->
+                val seen = mutableSetOf<String>()
+                val toRemove = mutableListOf<Int>()
+                for (i in stack.lastIndex downTo 0) {
+                    val entry = stack[i]
+                    if (entry is Sheet && !seen.add(entry.toString())) {
+                        toRemove.add(i)
+                    }
+                }
+                if (toRemove.isNotEmpty()) {
+                    Timber.w("Duplicate Sheet keys detected, sanitizing backstack")
+                    Snapshot.withMutableSnapshot {
+                        toRemove.forEach { navigator.backStack.removeAt(it) }
+                    }
+                }
+            }
+    }
+
     NavDisplay(
         backStack = navigator.backStack,
         onBack = onBack ?: {

ui/navigation/src/main/kotlin/com/getcode/navigation/core/CodeNavigator.kt

@@ -99,6 +99,9 @@ data class CodeNavigator(
             NavOptions.PopUpTo.ClearAll -> {
                 Snapshot.withMutableSnapshot {
                     if (currentRouteKey != route) {
+                        if (route is Sheet) {
+                            backStack.removeAll { it is Sheet && it.toString() == route.toString() }
+                        }
                         backStack.add(route)
                         // Remove all entries before the new one
                         while (backStack.size > 1) {