UI Controller

UI controllers are input adapters between raw canvas/browser events and grid behavior APIs.

Controller responsibilities:

1. Hit testing and gesture interpretation.
2. Mapping input intent to behavior/component actions.
3. Preserving interaction priority (for example resize vs select vs click).
4. Triggering focused invalidation/repaint updates.

Controllers are composable through the UI-controller manager, allowing applications to replace or extend specific interaction modes without rewriting core grid logic.