Custom Hierarchy background rect lines

How can I draw bg rect lines to seperate elements in Hierarchy window? Like in this plugin for example:

I tried with a function on EditorApplication.hierarchyWindowItemOnGUI event, but I have to draw all lines at one time so I'm using bool redraw to know if I should redraw bg this time.

 ...
 EditorApplication.update += Update;
 EditorApplication.hierarchyWindowItemOnGUI += DrawHierarchyBG;
 ...
 static void Update()
 {
         needRedraw = true;
 }
 
 static void DrawHierarchyBG(int instanceId, Rect selectionRect)
 {
         if (needRedraw)
         {
             needRedraw = false;
             int numberOfLines = Screen.height / 16;
 
             for (int i = 0; i < numberOfLines; i+=2)
             {
                 GUI.backgroundColor = new Color(.5f, .5f, .5f, .1f);
                 Rect grayRect = new Rect(0, 16 * i, Screen.width, 16);
                 GUI.Box(grayRect, "");
                 GUI.backgroundColor = Color.white;
             }
         }
 }

It works ok but flicker sometimes, for example if you move some other editor window, or resize hierarchy. Sometimes it flicker without reason at all. Also it's bad that I draw above hierarchy elements not below them. There should be a better way. Thank you!