Unity function to get the main editor window's position.

GetMainWindowPosition.cs

// This contains no error checking, which you want for such fragile code.
// You also probably want to cache `mainWindow` if you call this often.
static Rect GetMainWindowPosition()
{
    var containerWindowType = typeof(EditorWindow).Assembly.GetType("UnityEditor.ContainerWindow");
    var showModeField = containerWindowType.GetField("m_ShowMode", BindingFlags.Instance | BindingFlags.NonPublic);
    var mainWindow = Resources.FindObjectsOfTypeAll(containerWindowType).First(window => (int)showModeField.GetValue(window) == 4);
    var positionProperty = mainWindow.GetType().GetProperty("position", BindingFlags.Instance | BindingFlags.Public);
    return (Rect)positionProperty.GetValue(mainWindow, null);
}