Remove the brightness dip in screen transition crossfades

- Fading the outgoing and incoming screens simultaneously makes content
  present on both (the camera FAB) dip to ~75% opacity mid-transition
- Fade only one layer: hold the outgoing destination opaque under the
  fading-in incoming one, and reveal the destination instantly under the
  fading-out one on back

app/src/main/java/com/example/onecamera/navigation/OneCameraNavGraph.kt

@@ -67,8 +67,14 @@ fun OneCameraNavGraph(navController: NavHostController) {
         navController = navController,
         startDestination = FoldersRoute,
         enterTransition = { if (transitionsEnabled) fadeIn(tween(250)) else EnterTransition.None },
-        exitTransition = { if (transitionsEnabled) fadeOut(tween(250)) else ExitTransition.None },
-        popEnterTransition = { if (transitionsEnabled) fadeIn(tween(250)) else EnterTransition.None },
+        exitTransition = {
+            if (transitionsEnabled) {
+                ExitTransition.KeepUntilTransitionsFinished
+            } else {
+                ExitTransition.None
+            }
+        },
+        popEnterTransition = { EnterTransition.None },
         popExitTransition = { if (transitionsEnabled) fadeOut(tween(250)) else ExitTransition.None },
     ) {
         composable<FoldersRoute> {