So frustrating. An internal constant not accessible was added: internal const float kPrefixPaddingRight = 2; which PrefixLabel uses to return the control rect. I know it's a small thing, but a lot of my custom editor scripts (and even Unity editor scripts) have fields offset by 2px compared to other fields. For example, the LocalizedString drawer from Unity Localization package: Probably just using EditorGUIUtility.labelWidth to offset the control rect, because that's the way it's always been! At least expose kPrefixPaddingRight or modify the EditorGUIUtility.labelWidth property.