Binding to IsChecked property doesn't work in TwoWay mode (ConvertBack doesn't work).

Declined.
IsChecked / CheckState are supported only in xaml-defined RadTreeViews. In databinding scenarios use the approach described here =>
http://www.telerik.com/help/wpf/radtreeview-howto-tri-state-mvvm.html

a) Using WCF RIA Services
b) Using QueryableDomainServiceCollectionView
c) Using TreeView with above two
d) Including LoadOnDemand
e) Using "Prepared Item"

We have a RadTreeView with "Visibility" property set to "Hidden".
We add dynamically RadTreeViewItems to the RadTreeView.
If we change the "Visibility" to "Visible" - the RadTreeView is NOT displayed.

Originally this item is logged for scenario with XBAP. Currently we are unable to reproduce such issue with WPF 4.0 and latest version of our controls.
Please open a new support thread if you encounter similar issues at your side.

RadTreeView is virtualized and SelectionMode is set to "Extended". All ViewModels are selected and IsSelected of TreeViewitems is 2-way bound to IsSelected in the ViewModels. When clicking a TreeViewitem with Mouse, all other items are deselected. However, when scrolling we see that items are being prepared are not deselected - they are still selected.

This is due to a Limitation of the Virtualization mechanism - it does not know about the existance of the ViewModels and does not work with them. Virtualization works only with the Containers - the RadTreeViewItem.

Modify the SelectionChanged event logic so that the event is only fired once - exposing both the RemovedItems and AddedItems collections.

Improve selection performance.

The scroll position resets when the measure of the treeview panel is fired for some reason.

For example, when the RadTreeView is placed inside a RadPanelBarItem and the RadPanelBar's ScrollViewer is disabled, resizing the RadPanelBar resets the RadTreeView scroll position. 

The IsDragPreviewEnabled and IsDragTooltipEnabled properties don't work with the "NEW" DragDropExecutionMode.

Ability to validate if there are selected,checked, expanded items, if a node has items

Cannot properly scroll when using StackPanel as ItemsPanel.

When IsVirtulizing is set to true and there are many items in the tree view pressing several times PageUp key brings the view port to the beginning of the tree rather to the previous page.

Repro actions:
1) Hit the "End" key to go to the bottom
2) Hit the "Home" key to go back to the top
3) Mouse wheel does not move the scroll
4) The scroll arrows don't work either
5) They keyboard arrows don't work either, the selected item is displayed but they scroll bar does not move

See this video for reference.

Support DragCue and vertical PreviewDropLine when TreeView renders its items horizontally

When you tab through your application you can observe that when the focus moves to the TreeView it will focus the first TreeViewItem. However, when you change the ItemsSource collection of the TreeView at run-time you won't be able to tab through the TreeView or its first TreeViewItem at all.

Fixed in LIB version: 2014.2.0915

When the IsLineEnabled property is set to True and there are more than 3 levels of hierarchy, the lines look jagged.

This will solve scenarios in which user wants to add new items somewhere (inner level, outside the ViewPort ) in virtualized treeview and then wants to begin edit these items (or focus them , select them). 
There is currently no mechanism to detect when the new items are ready (measured / completely generated), ItemsPrepared for example passes before the OnApplyTemplate of the RadTreeViewItem.

We have RadContextMenu in RadTreeView when we press the App key or shift+F10 the context menu opens as intended but it triggers the parent TreeViewItem hover effect

Scenario: Databound TreeView with IsSelected bindings between item and container. Adding a selected data object to the ItemsSource does not fire the Selected event.

Allow cancelling a drag operation originating from certain items - basically the customers need to disable the drag operation on some items.
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Here is a possible solution with handling DragInitialize
 
DragDropManager.AddDragInitializeHandler(this.tree, this.OnDragInitialize, true);

 private void OnDragInit(object sender, DragInitializeEventArgs e)
        {
            if (((e.OriginalSource as RadTreeViewItem).Header == "B1")
            {
                e.Data = null;
                e.DragVisual = null;
            }

The Horizontal ScrollBar cannot be hidden/disabled in certain situatations, for example when TreeView is inside RadPanelBarItem