protected virtual ReorderableList GatherDetails(SerializedProperty property)

This method gets called by every Animancer.Editor.ManualMixerTransitionDrawer.GetPropertyHeight(UnityEditor.SerializedProperty,UnityEngine.GUIContent) and Animancer.Editor.ManualMixerTransitionDrawer.OnGUI(UnityEngine.Rect,UnityEditor.SerializedProperty,UnityEngine.GUIContent) call since Unity uses the same UnityEditor.PropertyDrawer instance for each element in a collection, so it needs to gather the details associated with the current property.