function BeginWindows () : void

Description

Mark the beginning area of all popup windows.

GUI.Window behaves somewhat differently in the editor than inside your games. In games, GUI.Window pops up a window on your screen. In the editor, GUI.Window shows a subwindow inside one of your editor windows. Begin/EndWindows is used to determine where these can go. You need to have all calls to GUI.Window or GUILayout.Window inside a BeginWindows / EndWindows pair. Like this:

Simple editor Window with a window and a button inside.

class GUIWindowDemo extends EditorWindow {
    // The position of the window
    var windowRect = Rect (100,100,200,200);

    // Main GUI Function
    function OnGUI () {
        // Begin Window
        BeginWindows ();
        
        // All GUI.Window or GUILayout.Window must come inside here
        windowRect = GUILayout.Window (1, windowRect, DoWindow, "Hi There");        
        
        // Collect all the windows between the two.
        EndWindows ();            
    }
    
    // The window function. This works just like ingame GUI.Window
    function DoWindow () {
        GUILayout.Button ("Hi");
        GUI.DragWindow ();        
    }
    
    // Add menu item to show this demo.
    @MenuItem ("Test/GUIWindow Demo")
    static function Init () {
        EditorWindow.GetWindow (GUIWindowDemo);
    }
}

The placement of the BeginWindows / EndWindows pair determines where popup windows will appear; all windows are clipped to the clipping area defined by GUI.BeginGroup or GUI.BeginScrollview. A small example of that

Simple editor window with a window and a button inside using scroll bars.

class GUIWindowDemo2 extends EditorWindow {
    // The position of the window
    var windowRect = Rect (100,100,200,200);

    // Scroll position
    var scrollPos = Vector2.zero;

    function OnGUI () {
        // Set up a scroll view
        scrollPos = GUI.BeginScrollView (
            new Rect (0, 0, position.width, position.height), 
            scrollPos, 
            new Rect (0, 0, 1000, 1000)
        );

        // Same code as before - make a window. Only now, it's INSIDE the scrollview
        BeginWindows ();    
        windowRect = GUILayout.Window (1, windowRect, DoWindow, "Hi There");                
        EndWindows ();
        
        // Close the scroll view
        GUI.EndScrollView ();
    }
    
    function DoWindow () {
        GUILayout.Button ("Hi");
        GUI.DragWindow ();        
    }

    @MenuItem ("Test/GUIWindow Demo 2")
    static function Init () {
        EditorWindow.GetWindow (GUIWindowDemo2);
    }
}