Don't allow selection of non-expanded TreeViewItems.

And remove child selection when a `TreeViewItem` is un-expanded.

This is necessary because we don't get enough information about a materialized `TreeViewItem` to select it when materialized, so the `SelectionModel` and `TreeViewItem` selection state gets out of sync.

src/Avalonia.Controls/TreeView.cs

@@ -4,6 +4,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
+using System.Reactive.Linq;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Utils;
@@ -395,7 +396,14 @@ namespace Avalonia.Controls
         private void OnSelectionModelChildrenRequested(object sender, SelectionModelChildrenRequestedEventArgs e)
         {
             var container = ItemContainerGenerator.Index.ContainerFromItem(e.Source) as ItemsControl;
-            e.Children = container?.GetObservable(ItemsProperty);
+
+            if (container is object)
+            {
+                e.Children = Observable.CombineLatest(
+                    container.GetObservable(TreeViewItem.IsExpandedProperty),
+                    container.GetObservable(ItemsProperty),
+                    (expanded, items) => expanded ? items : null);
+            }
         }
 
         private TreeViewItem GetContainerInDirection(

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -130,6 +130,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var item = tree[0].Children[1].Children[0];
             var container = (TreeViewItem)target.ItemContainerGenerator.Index.ContainerFromItem(item);
@@ -157,6 +158,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var item = tree[0].Children[1].Children[0];
             var container = (TreeViewItem)target.ItemContainerGenerator.Index.ContainerFromItem(item);
@@ -188,6 +190,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var item1 = tree[0].Children[1].Children[0];
             var container1 = (TreeViewItem)target.ItemContainerGenerator.Index.ContainerFromItem(item1);
@@ -225,6 +228,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var rootNode = tree[0];
 
@@ -264,6 +268,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var rootNode = tree[0];
 
@@ -297,6 +302,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var rootNode = tree[0];
 
@@ -330,6 +336,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var rootNode = tree[0];
 
@@ -376,6 +383,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var item = tree[0].Children[1].Children[0];
             var container = (TreeViewItem)target.ItemContainerGenerator.Index.ContainerFromItem(item);
@@ -402,6 +410,7 @@ namespace Avalonia.Controls.UnitTests
 
             CreateNodeDataTemplate(target);
             ApplyTemplates(target);
+            ExpandAll(target);
 
             var item = tree[0].Children[1].Children[0];
 
@@ -579,6 +588,7 @@ namespace Avalonia.Controls.UnitTests
 
                 CreateNodeDataTemplate(target);
                 ApplyTemplates(target);
+                ExpandAll(target);
 
                 var item = data[0].Children[0];
                 var node = target.ItemContainerGenerator.Index.ContainerFromItem(item);
@@ -614,6 +624,7 @@ namespace Avalonia.Controls.UnitTests
 
                 CreateNodeDataTemplate(target);
                 ApplyTemplates(target);
+                ExpandAll(target);
 
                 var rootNode = tree[0];
 
@@ -651,6 +662,7 @@ namespace Avalonia.Controls.UnitTests
 
                 CreateNodeDataTemplate(target);
                 ApplyTemplates(target);
+                ExpandAll(target);
 
                 var rootNode = tree[0];
 
@@ -697,6 +709,7 @@ namespace Avalonia.Controls.UnitTests
 
                 CreateNodeDataTemplate(target);
                 ApplyTemplates(target);
+                ExpandAll(target);
 
                 var rootNode = tree[0];