fix: handle hover styling for field containers

packages/ui/lib/field-root-container-classes.ts

@@ -5,6 +5,18 @@ export const FIELD_ROOT_CONTAINER_CLASS_NAME = `${FIELD_ROOT_CONTAINER_SHARED_CL
 
 export const FIELD_ROOT_CONTAINER_PROBE_CLASS_NAME = `field--FieldRootContainerProbe ${FIELD_ROOT_CONTAINER_SHARED_CLASS_NAME}`;
 
+/**
+ * Class embedders target to style a field's hover state, e.g.
+ * `.field--FieldRootContainerHover { background-color: ... }`.
+ *
+ * Canvas-rendered fields can't use the `:hover` pseudo-class (there's no real
+ * DOM element to hover), so the hover probe applies this class permanently to
+ * read the hovered cascade synchronously via `getComputedStyle`. Using a real
+ * class instead of `:hover` keeps resolution deterministic — no stylesheet
+ * parsing, pseudo-class synthesis, or source-order/specificity guesswork.
+ */
+export const FIELD_ROOT_CONTAINER_HOVER_CLASS_NAME = 'field--FieldRootContainerHover';
+
 /**
  * Selector for the element the probe is appended into when reading computed
  * field styles. It must be an ancestor of where real fields render so the probe