MainScreen as an Orchestrator

The MainScreen is the most complex UI component in the app. Unlike standard features, it acts as a container and orchestrator for the top-level navigation.

Responsibilities

1. Bottom Navigation Management: It holds the BottomNavigationBar and manages switching between tabs (e.g., Groups, Expenses, Balances) using a persistent BottomNavigationController.
2. Multi-Stack State Preservation: It maintains a separate NavHostController for each tab. It uses MainViewModel to save and restore the Bundle state of each controller when switching tabs, ensuring the back stack is preserved (e.g., if you drill down into an expense details screen, switch tabs, and come back, the details screen is still there).
3. Dynamic UI Construction: It observes the current inner route of the active tab to dynamically decide which ScreenUiProvider (TopBar/FAB) to hoist to the parent Scaffold.
4. Visual Effects & Transitions: It initializes the SharedTransitionLayout to enable shared element transitions between screens and manages the HazeState for glassmorphism effects on the bottom bar.

Why not use FeatureScaffold?

We use FeatureScaffold for "leaf" screens (simple screens with one job). MainScreen cannot use it because:

• Persistent Lifecycle: It is not destroyed when changing tabs; it acts as the host.
• Complex Logic: It needs to calculate logic based on "Which tab is active?" combined with "Which screen is active inside the tab?" to render the correct TopBar.
• Multiple NavHosts: It manages a map of multiple NavHostController instances (one per tab) to keep stacks independent, whereas FeatureScaffold assumes a single linear flow.

Architecture

MainScreen acts as the "Host" that swaps content dynamically while keeping the Scaffold (TopBar/BottomBar/FAB) persistent, providing a smooth user experience.

• Window Insets: It consumes WindowInsets manually to allow content (like lists) to draw behind the transparent Bottom Navigation Bar, creating an edge-to-edge experience.
• Composition Locals: It provides LocalTabNavController and LocalSharedTransitionScope to its children, enabling them to trigger global navigation actions or shared animations.