Why are GUILayout controls started before the BeginArea they are wrapped in?

I'm working on an editor script and I've got this bit of code:

Code (csharp):

1. void OnGUI()
2. {
3.     EditorGUILayout.BeginHorizontal(EditorStyles.toolbar);
4.     EditorGUILayout.Space();
5.     EditorGUILayout.EndHorizontal();
6.  
7.     var _sidebar = GUILayoutUtility.GetLastRect();
8.     _sidebar.x = Screen.width - 400;
9.     _sidebar.y = _sidebar.height;
10.     _sidebar.width = 300;
11.     _sidebar.height = Screen.height - _sidebar.height;
12.  
13.     GUI.Box(_sidebar, "");
14.     GUILayout.BeginArea(_sidebar);
15.     GUILayout.Button("Area Test");
16.     GUILayout.EndArea();
17. }

For some reason the Button appears above the area specified by the call to BeginArea. Specifically, it behaves as if the origin of the button was bottom left, instead of top left as it should be.

Here's a screenshot of what's going on:

 

Just have seen your problem and tried some things out.

The Problem ist the call of GUILayoutUtiliy.GetLastRect() . Made a DebugLog after the call and printing the _sidebar.height and it was changing between 1 und 18 the hole time.

So instead of
_sidebar.y = _sidebar.height -> _sidebar.y = 18;
_sidebar.height = window.position.height - _sidebar.height; -> _sidebar.height = window.position.height - 18;

and your problem was fixed.

Additionally i would prefer instead of Screen.width and Screen.height to use

var window = EditorWindow.GetWindow(typeof(yourEditorScript));

and than use

window.position.width
window.position.height

instead. Maybe this will be helpfull in future. If it even makes any difference.

Best regards

Code (CSharp):

1. private const int SPACE = 18;
2.  
3. void OnGUI()
4. {
5.     EditorGUILayout.BeginHorizontal(EditorStyles.toolbar);
6.     EditorGUILayout.BeginVertical();
7.      GUILayout.Space(SPACE);
8.     EditorGUILayout.EndVertical();
9.     EditorGUILayout.EndHorizontal();
10.  
11.     var window = EditorWindow.GetWindow(typeof(myEditorScript));
12.              
13.      var _sidebar = new Rect();
14.      _sidebar.x = window.position.width - 400;
15.      _sidebar.y = SPACE;
16.      _sidebar.width = 300;
17.      _sidebar.height = window.position.height - SPACE;
18.  
19.     GUI.Box(_sidebar, "");
20.     GUILayout.BeginArea(_sidebar);
21.     GUILayout.Button("Area Test");
22.     GUILayout.EndArea();
23. }