UMG and CommonUI

Ownership boundary

This skill owns widget lifecycle, screen layers, focus, navigation, and UI input routing. Route action, context, trigger, modifier, and key-profile semantics to unreal-enhanced-input, authoritative multiplayer state to unreal-replication, and durable slot/schema/migration policy to unreal-save-load.

Select the UI stack

• Use UMG for every game UI; it is the base authoring layer.
• Add CommonUI when the product needs complex layered screens, robust gamepad navigation, cross-platform input prompts, or consistent back/input routing.
• Do not add CommonUI solely for a small mouse-driven HUD or a single simple menu.
• Treat UMG Viewmodel/MVVM as Beta in UE 5.8. Use it when push binding and UI/backend separation justify the maturity risk; otherwise use explicit delegates and setters.
• In UE 5.8 prefer the unified CommonUI + Enhanced Input workflow. Do not copy older tutorials that require a parallel UI action-data-table vocabulary unless maintaining legacy content.

Read references/system-selector.md before choosing.

Preserve the presentation boundary

1. Keep authoritative gameplay state in gameplay systems, never in widgets.
2. Expose a small local presentation model or Viewmodel per local player.
3. Push changes into the UI with delegates, events, or FieldNotify.
4. Let widgets submit semantic intent; let gameplay code validate and execute it.
5. Replicate model state, not widget instances. Create and update UI locally.

Read references/architecture-lifecycle.md and route network-state design to $unreal-replication.

Build in dependency order

1. Identify local-player ownership, screen layers, and the authority boundary.
2. Establish one root layout and named HUD, menu, modal, and notification layers.
3. Make screens activatable only when they own an input/focus boundary.
4. Define activation, back, input mode, mouse capture, and desired-focus behavior.
5. Build layouts with anchors, containers, DPI scaling, and Safe Zones.
6. Push state changes; avoid Tick and raw property bindings.
7. Virtualize long collections and load expensive, infrequent screens on demand.
8. Test keyboard/mouse, gamepad, touch if supported, split screen, resolutions, safe zones, localization expansion, and accessibility settings.
9. Profile with Slate Insights/Unreal Insights before applying invalidation or retainers.

Use references/commonui-routing.md, references/input-focus.md, and references/responsive-accessibility.md.

Hard rules

• Never store authoritative gameplay state only in a widget.
• Never replicate widgets or use a widget as a client-to-server RPC owner.
• Use an owning LocalPlayer/PlayerController; prefer AddToPlayerScreen for player UI.
• Do not assume Construct runs once; use initialization and activation hooks intentionally.
• Do not make every CommonUI widget activatable.
• Every gamepad screen must define a valid initial focus target and a back path.
• Never encode critical information by color alone; support readable text scaling.
• Avoid raw property binding, widget Tick, and nested Canvas Panels in production UI.
• Do not add Invalidation Boxes or Retainer Panels without profiling and change-frequency analysis.

Read references/binding-mvvm.md, references/performance.md, and references/patterns-troubleshooting.md. See references/sources.md for the UE 5.8 source trail.