dragtab is no longer working (Unity 2019). How can I make my Inspector look like this?

Here's my old custom inspector code, which displays nice before Unity 2019.

Code (CSharp):

1.         var text = "Music Ducking";
2.         text = "<b><size=11>" + text + "</size></b>";
3.  
4.         if (state) {
5.             text = "\u25BC " + text;
6.         } else {
7.             text = "\u25BA " + text;
8.         }
9.  
10.         if (!GUILayout.Toggle(true, text, "dragtab", GUILayout.MinWidth(20f))) {
11.             state = !state;
12.         }
13.  

Looks like this when used in a couple consecutive sections:



But in Unity 2019 there is no inner background, no rounded corner, it's just the text on the gray background.

I'm not sure how that code even works, I copied it from some other plugin years ago, but how can I make nice looking expandable areas like this that work in Unity 2019+?