UE UI: UMG, Slate, and Common UI

Target engine: UE 5.8. APIs below are verified against the 5.8 headers; older forms are listed under "Deprecated — do not use".

UMG is the runtime UI framework (UMG module, Engine/Source/Runtime/UMG/Public) built on Slate (Slate, SlateCore). Common UI (plugin, production in 5.8) adds cross-platform input routing, activation stacks and platform-aware buttons. Model View Viewmodel (plugin, Beta in 5.8) adds declarative data binding on top of FieldNotification.

Context

Read .agents/ue-project-context.md if it exists (module names, conventions, enabled plugins, GAS/networking setup). Do not stop if it is missing.

Identify the area from the request and the codebase. Ask only when two plausible readings would produce different code.