Annotation.cs

using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;
#endif

/// <summary>
/// A component for adding annotations to GameObjects in a scene
/// </summary>
[AddComponentMenu("Annotation")]
internal sealed class Annotation : MonoBehaviour
{
    [SerializeField, TextArea] private string _text;

#if UNITY_EDITOR
    [CustomEditor(typeof(Annotation), true), CanEditMultipleObjects]
    internal sealed class AnnotationEditor : Editor
    {
        private SerializedProperty _text;
        
        private bool _isEditable;
        private string _lastFocusedControl;

        private void OnEnable()
        {
            // Get text property and begin editing if annotation has not been set
            _text = serializedObject.FindProperty(nameof(_text));
            if (string.IsNullOrWhiteSpace(_text.stringValue))
            {
                Edit();
            }
        }

        public override void OnInspectorGUI()
        {
            serializedObject.Update();

            // Draw the non-editable annotation
            if (!_isEditable)
            {
                var style = new GUIStyle(EditorStyles.label) { wordWrap = true, richText = true };
                EditorGUILayout.LabelField(GUIContent.none, new GUIContent(_text.stringValue), style);

                // Add context menu to allow editing
                var e = Event.current;
                if (e.type == EventType.ContextClick && GUILayoutUtility.GetLastRect().Contains(e.mousePosition))
                {
                    ShowContextMenu();
                    e.Use();
                }

                return;
            }

            // Draw editable annotation
            using (var check = new EditorGUI.ChangeCheckScope())
            {
                var style = new GUIStyle(EditorStyles.textArea) { wordWrap = true, richText = false };
                GUI.SetNextControlName(nameof(_text));
                _text.stringValue = EditorGUILayout.TextArea(_text.stringValue, style);

                serializedObject.ApplyModifiedProperties();
                if (check.changed)
                {
                    EditorUtility.SetDirty(target);
                }

                // Disable editing when losing focus
                var currentFocusedControl = GUI.GetNameOfFocusedControl();
                if (_lastFocusedControl == nameof(_text) && currentFocusedControl != nameof(_text))
                {
                    _isEditable = false;
                }

                _lastFocusedControl = currentFocusedControl;
            }
        }

        private void ShowContextMenu()
        {
            var menu = new GenericMenu();
            menu.AddItem(new GUIContent("Edit"), false, Edit);
            menu.ShowAsContext();
        }

        private void Edit()
        {
            _isEditable = true;
        }
    }
#endif
}