Gets tree syncing working, updates the GetApplicationTrees to still return a single Root since we require this everywhere for JS, fixes full screen section and makes users a full screen section (and reduces unecessary code), removes code to avoid breaking changes from v7

src/Umbraco.Web/Models/Trees/SectionRootNode.cs

@@ -1,55 +1,154 @@
-using System.Linq;
+using System.Globalization;
+using System.Linq;
 using System.Runtime.Serialization;
-using Umbraco.Core;
 
 namespace Umbraco.Web.Models.Trees
 {
     /// <summary>
-    /// A special tree node that represents the section root node for any section.
+    /// A tree node that represents various types of root nodes
     /// </summary>
     /// <remarks>
+    /// <para>
+    /// A <see cref="TreeRootNode"/> represents:
+    /// * The root node for a section containing a single tree
+    /// * The root node for a section containing multiple sub-trees
+    /// * The root node for a section containing groups of multiple sub-trees
+    /// * The group node in a section containing groups of multiple sub-trees
+    /// </para>
+    /// <para>
     /// This is required to return the tree data for a given section. Some sections may only contain one tree which means it's section
     /// root should also display a menu, whereas other sections have multiple trees and the section root shouldn't display a menu.
-    ///
-    /// The section root also contains an explicit collection of children.
+    /// </para>
+    /// <para>
+    /// The root node also contains an explicit collection of children.
+    /// </para>
     /// </remarks>
     [DataContract(Name = "node", Namespace = "")]
-    public sealed class SectionRootNode : TreeNode
+    public sealed class TreeRootNode : TreeNode
     {
-        public static SectionRootNode CreateMultiTreeSectionRoot(string nodeId, TreeNodeCollection children)
-        {
-           var sectionRoot = new SectionRootNode(nodeId, "", "")
-                {
-                    IsContainer = true,
-                    Children = children
-                };
+        private static readonly string RootId = Core.Constants.System.Root.ToString(CultureInfo.InvariantCulture);
+        private bool _isGroup;
 
-            //some metadata as to whether or not this section contains any trees
-            sectionRoot.AdditionalData["containsTrees"] = children.Any();
+        /// <summary>
+        /// Creates a group node for grouped multiple trees
+        /// </summary>
+        /// <param name="children"></param>
+        /// <returns></returns>
+        public static TreeRootNode CreateGroupNode(TreeNodeCollection children)
+        {
+            var sectionRoot = new TreeRootNode(RootId, string.Empty, string.Empty)
+            {
+                IsGroup = true,
+                Children = children
+            };
 
             return sectionRoot;
         }
 
-        public static SectionRootNode CreateSingleTreeSectionRoot(string nodeId, string getChildNodesUrl, string menuUrl, string title, TreeNodeCollection children)
+        /// <summary>
+        /// Creates a section root node for grouped multiple trees
+        /// </summary>
+        /// <param name="children"></param>
+        /// <returns></returns>
+        public static TreeRootNode CreateGroupedMultiTreeRoot(TreeNodeCollection children)
         {
-            return new SectionRootNode(nodeId, getChildNodesUrl, menuUrl)
+            var sectionRoot = new TreeRootNode(RootId, string.Empty, string.Empty)
+            {
+                IsContainer = true,
+                Children = children,
+                ContainsGroups = true
+            };
+
+            return sectionRoot;
+        }
+
+        /// <summary>
+        /// Creates a section root node for non-grouped multiple trees
+        /// </summary>
+        /// <param name="children"></param>
+        /// <returns></returns>
+        public static TreeRootNode CreateMultiTreeRoot(TreeNodeCollection children)
+        {
+            var sectionRoot = new TreeRootNode(RootId, string.Empty, string.Empty)
+            {
+                IsContainer = true,
+                Children = children
+            };
+
+            return sectionRoot;
+        }
+
+        /// <summary>
+        /// Creates a section root node for a section with a single tree
+        /// </summary>
+        /// <param name="nodeId"></param>
+        /// <param name="getChildNodesUrl"></param>
+        /// <param name="menuUrl"></param>
+        /// <param name="title"></param>
+        /// <param name="children"></param>
+        /// <returns></returns>
+        public static TreeRootNode CreateSingleTreeRoot(string nodeId, string getChildNodesUrl, string menuUrl, string title, TreeNodeCollection children)
+        {
+            return new TreeRootNode(nodeId, getChildNodesUrl, menuUrl)
             {
                 Children = children,
                 Name = title
             };
         }
 
-        private SectionRootNode(string nodeId, string getChildNodesUrl, string menuUrl)
+        /// <summary>
+        /// Private constructor
+        /// </summary>
+        /// <param name="nodeId"></param>
+        /// <param name="getChildNodesUrl"></param>
+        /// <param name="menuUrl"></param>
+        private TreeRootNode(string nodeId, string getChildNodesUrl, string menuUrl)
             : base(nodeId, null, getChildNodesUrl, menuUrl)
         {
             //default to false
             IsContainer = false;
         }
 
+        /// <summary>
+        /// Will be true if this is a multi-tree section root node (i.e. contains other trees)
+        /// </summary>
         [DataMember(Name = "isContainer")]
         public bool IsContainer { get; private set; }
 
+        /// <summary>
+        /// True if this is a group root node
+        /// </summary>
+        [DataMember(Name = "isGroup")]
+        public bool IsGroup
+        {
+            get => _isGroup;
+            private set
+            {
+                //if a group is true then it is also a container
+                _isGroup = value;
+                IsContainer = true;
+            }
+        }
+
+        /// <summary>
+        /// True if this root node contains group root nodes
+        /// </summary>
+        [DataMember(Name = "containsGroups")]
+        public bool ContainsGroups { get; private set; }
+
+        /// <summary>
+        /// The node's children collection
+        /// </summary>
         [DataMember(Name = "children")]
         public TreeNodeCollection Children { get; private set; }
+
+        /// <summary>
+        /// Returns true if there are any children
+        /// </summary>
+        /// <remarks>
+        /// This is used in the UI to configure a full screen section/app
+        /// </remarks>
+        [DataMember(Name = "containsTrees")]
+        public bool ContainsTrees => Children.Count > 0;
     }
 }