PropertyDrawer OnGUI Event.Current - tutorial overview?

I'm getting into the weeds of PropertyDrawers/OnGUI, and running into a variety of errors relating to :

ArgumentException: GUILayout: Mismatched LayoutGroup.dragPerform
ArgumentException: GUILayout: Mismatched LayoutGroup.MouseUp
ArgumentException: GUILayout: Mismatched LayoutGroup.Repaint
ArgumentException: GUILayout: Mismatched LayoutGroup.ignore
and a large variety of others, all in
Mismatched LayoutGroup.*

The code in the inspector is all working, and I'm finding hints about the relationship between OnGUI and Event.Current mentioned. What I'm not finding is a good overview of how they relate. Particularly what you can do during each event type, or what you should do.

Any tutorials on this, so I don't keep asking vague questions showing that I'm missing the point.

- Thanks.