SerializableDictionaryDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(SerializableDictionary<,>))]
public class SerializableDictionaryDrawer : PropertyDrawer
{
    private const float Padding = 2f;

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        EditorGUI.BeginProperty(position, label, property);

        position.height = EditorGUIUtility.singleLineHeight;

        SerializedProperty keysProperty = property.FindPropertyRelative("keys");
        SerializedProperty valuesProperty = property.FindPropertyRelative("values");

        int size = keysProperty.arraySize;

        float halfWidth = (position.width - Padding) * 0.5f;

        Rect leftRect = new Rect(position.x, position.y, halfWidth, position.height);
        Rect rightRect = new Rect(position.x + halfWidth + Padding, position.y, halfWidth, position.height);

        EditorGUI.LabelField(leftRect, "Keys");
        EditorGUI.LabelField(rightRect, "Values");

        // EditorGUI.indentLevel++;
        for (int i = 0; i < size; i++)
        {
            Rect keyRect = new Rect(position.x, position.y + (position.height + Padding) * (i + 1), halfWidth, position.height);
            Rect valueRect = new Rect(position.x + halfWidth + Padding, position.y + (position.height + Padding) * (i + 1), halfWidth, position.height);

            EditorGUI.PropertyField(keyRect, keysProperty.GetArrayElementAtIndex(i), GUIContent.none);
            EditorGUI.PropertyField(valueRect, valuesProperty.GetArrayElementAtIndex(i), GUIContent.none, true);
        }
        // EditorGUI.indentLevel--;

        Rect addButtonRect = new Rect(position.x, position.y + (position.height + Padding) * (size + 1), position.width, position.height);
        if (GUI.Button(addButtonRect, "Add Key-Value Pair"))
        {
            Debug.Log("Adding kv pair");
            keysProperty.arraySize++;
            valuesProperty.arraySize++;
        }

        EditorGUI.EndProperty();
    }

    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        SerializedProperty keysProperty = property.FindPropertyRelative("keys");
        int size = keysProperty.arraySize;

        return EditorGUIUtility.singleLineHeight * (size + 1) + Padding * size;
    }
}