When the IsHidden property of a RadPane within a ToolWindow is set to True and then to false in quick succession, the ToolWindow is not shown. 

As a workaround, you can disable the animation of the ToolWindow:
<Style TargetType="telerik:ToolWindow">
<Setter Property="telerik:AnimationManager.IsAnimationEnabled" Value="False" />
</Style>

The above style need to be placed in the resources of the App.xaml in order to take effect on the ToolWindows, since they are displayed in a separated visual tree.

The exception is reproduced when multiple toolwindows with nested owners are floated and they are reloaded by re-merging the dictionaries in App.xaml in a NoXaml scenario.

When the RadDocking Items are reordered during runtime and new RadSplitContainers/RadPaneGroups are generated, the save/load behavior is sometimes incorrect. 

Dynamically adding RadPane to PaneGroup, the header of the pane does not appear on the second added pane.

To workaround this the OnApplyTemplate() method can be called after the RadPane is added to the RadPaneGroup.

The close button in the document host is not higlighted (hovered-button-state-doc-host.png) as opposed to panes outside the DocumentHost area (hovered-button-state.png).

The workaround is to update the MouseOver visual state of the RadButtonDocumentCloseControlTemplate:

             <ControlTemplate x:Key="RadButtonDocumentCloseControlTemplate" TargetType="telerik:RadButton">
                <Grid x:Name="LayoutRoot" Background="Transparent" Width="18" Height="18">
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualState x:Name="Disabled">
                                <Storyboard>
                                    <DoubleAnimation Duration="0:0:0" To="0.3" Storyboard.TargetName="PathVisual" Storyboard.TargetProperty="Opacity"/>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Normal"/>
                            <VisualState x:Name="MouseOver">
                                <Storyboard>
                                    <DoubleAnimation Duration="0:0:0" To="1" Storyboard.TargetName="MouseOverVisual" Storyboard.TargetProperty="Opacity"/>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Pressed"/>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <Border x:Name="MouseOverVisual" Background="{telerik:Office2013Resource ResourceKey=EffectHighBrush}" Opacity="0"/>
                    <Path x:Name="PathVisual"
                    Width="8"
                    Height="8"
                    Data="M2.5,0 L4.5,0 4.5,2.5 7,2.5 7,4.5 4.5,4.5 4.5,7 2.5,7 2.5,4.5 0,4.5 0,2.5 2.5,2.5 z"
                    Stretch="Fill"
                    Fill="{TemplateBinding Background}"
                    RenderTransformOrigin="0.5,0.5"
                    SnapsToDevicePixels="True">
                        <Path.RenderTransform>
                            <RotateTransform Angle="-135"/>
                        </Path.RenderTransform>
                    </Path>
                </Grid>
            </ControlTemplate>
            <Style x:Key="CustomRadButtonDocumentCloseStyle" BasedOn="{StaticResource RadButtonDocumentCloseStyle}" TargetType="telerik:RadButton">
                <Setter Property="Template" Value="{StaticResource RadButtonDocumentCloseControlTemplate}"/>
            </Style>
            <ControlTemplate x:Key="RadPaneGroupDocumentHostControlTemplate" TargetType="telerik:RadPaneGroup">
                <Grid>
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualState x:Name="Disabled"/>
                            <VisualState x:Name="Normal"/>
                        </VisualStateGroup>
                        <VisualStateGroup x:Name="FocusStates">
                            <VisualState x:Name="Focused"/>
                            <VisualState x:Name="Unfocused"/>
                        </VisualStateGroup>
                        <VisualStateGroup x:Name="AutoCollapseStates">
                            <VisualState x:Name="SingleItem"/>
                            <VisualState x:Name="TwoOrMoreItems"/>
                        </VisualStateGroup>
                        <VisualStateGroup x:Name="PaneHeaderVisibilityStates">
                            <VisualState x:Name="PaneHeaderHidden"/>
                            <VisualState x:Name="PaneHeaderVisible"/>
                        </VisualStateGroup>
                        <VisualStateGroup x:Name="ResizerStates">
                            <VisualState x:Name="ResizerLeft">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="Visibility" Duration="0">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Visibility>Visible</Visibility>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="(Grid.RowSpan)" Duration="0">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <sys:Int32>3</sys:Int32>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="Placement" Duration="0">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Dock>Left</Dock>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <DoubleAnimation BeginTime="00:00:00" From="4" To="4" Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="MinWidth"/>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="ResizerTop">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="Visibility" Duration="0">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Visibility>Visible</Visibility>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="(Grid.ColumnSpan)" Duration="0">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <sys:Int32>2</sys:Int32>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="Placement" Duration="0">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Dock>Top</Dock>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                    <DoubleAnimation BeginTime="00:00:00" From="4" To="4" Storyboard.TargetName="DockResizer" Storyboard.TargetProperty="MinHeight"/>
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="HideResizer"/>
                        </VisualStateGroup>
                        <VisualStateGroup x:Name="DropDownDisplayStates">
                            <VisualState x:Name="DropDownButtonCollapsed"/>
                            <VisualState x:Name="DropDownButtonVisible">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DropDownButtonElement" Storyboard.TargetProperty="Visibility">
                                        <DiscreteObjectKeyFrame KeyTime="0">
                                            <DiscreteObjectKeyFrame.Value>
                                                <Visibility>Visible</Visibility>
                                            </DiscreteObjectKeyFrame.Value>
                                        </DiscreteObjectKeyFrame>
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition Height="*"/>
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="Auto"/>
                        <ColumnDefinition Width="*"/>
                    </Grid.ColumnDefinitions>
                    <dock:RadGridResizer x:Name="DockResizer" Placement="Left" ShowsPreview="True" Visibility="Collapsed"/>
                    <Grid x:Name="ItemsContainer" Grid.Row="1" Grid.Column="1">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>
                        <RepeatButton x:Name="LeftScrollButtonElement" Visibility="{Binding ComputedHorizontalScrollBarVisibility, ElementName=ScrollViewerElement}" Style="{StaticResource RepeatButtonScrollLeftStyle}"/>
                        <ScrollViewer x:Name="ScrollViewerElement" Grid.Column="1" HorizontalScrollBarVisibility="{Binding (ScrollViewer.HorizontalScrollBarVisibility), RelativeSource={RelativeSource TemplatedParent}}">
                            <ScrollViewer.Template>
                                <ControlTemplate TargetType="ScrollViewer">
                                    <ScrollContentPresenter x:Name="ScrollContentPresenter"/>
                                </ControlTemplate>
                            </ScrollViewer.Template>
                            <ItemsPresenter x:Name="ItemsPresenterElement"/>
                        </ScrollViewer>
                        <RepeatButton x:Name="RightScrollButtonElement"
                        Grid.Column="2"
                        Visibility="{Binding ComputedHorizontalScrollBarVisibility, ElementName=ScrollViewerElement}"
                        Style="{StaticResource RepeatButtonScrollRightStyle}"/>
                        <telerik:RadDropDownButton x:Name="DropDownButtonElement"
                        Grid.Column="3"
                        Visibility="Collapsed"
                        IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                        DropDownPlacement="Bottom"
                        DropDownButtonPosition="Left"
                        Style="{StaticResource DropDownButtonDocumentDropDownStyle}"
                        Focusable="False">
                            <telerik:RadDropDownButton.DropDownContent>
                                <telerikTabControl:DropDownMenu x:Name="DropDownMenuElement"
                                StaysOpen="False"
                                IconColumnWidth="0"
                                Placement="Bottom"
                                ItemTemplate="{TemplateBinding ItemDropDownContentTemplate}"
                                ItemTemplateSelector="{TemplateBinding ItemDropDownContentTemplateSelector}"
                                DisplayMemberPath="{TemplateBinding DropDownDisplayMemberPath}"/>
                            </telerik:RadDropDownButton.DropDownContent>
                        </telerik:RadDropDownButton>
                        <telerik:RadButton x:Name="CloseButton"
                        Grid.Column="4"
                        Command="telerik:RadDockingCommands.Close"
                        Style="{StaticResource CustomRadButtonDocumentCloseStyle}"
                        Visibility="{Binding IsEnabled, RelativeSource={RelativeSource Self}, Converter={StaticResource BooleanToVisibilityConverter}}"/>
                    </Grid>
                    <Grid Grid.Row="2" Grid.Column="1" Background="{TemplateBinding Background}" Margin="0 -1 0 0">
                        <Rectangle StrokeThickness="1" Stroke="{telerik:Office2013Resource ResourceKey=HighDarkBrush}"/>
                        <Rectangle Stroke="{telerik:Office2013Resource ResourceKey=AccentBrush}" StrokeThickness="1" Visibility="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedPane.IsActive, Converter={StaticResource BooleanToVisibilityConverter}}"/>
                        <Rectangle Margin="1" StrokeThickness="3" Stroke="{telerik:Office2013Resource ResourceKey=AccentBrush}" Opacity="0.3"/>
                        <Grid Margin="5">
                            <ContentPresenter x:Name="ContentElement"
                            Visibility="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=IsContentPreserved, Converter={StaticResource InvertedBooleanToVisibilityConverter}}"
                            ContentTemplate="{TemplateBinding SelectedContentTemplate}"
                            ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"/>
                            <Grid x:Name="ContentElementsPanel" Visibility="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=IsContentPreserved, Converter={StaticResource BooleanToVisibilityConverter}}"/>
                        </Grid>
                    </Grid>
                </Grid>
            </ControlTemplate>
            <Style BasedOn="{StaticResource RadPaneGroupStyle}" TargetType="telerik:RadPaneGroup">
                <Setter Property="DocumentHostTemplate" Value="{StaticResource RadPaneGroupDocumentHostControlTemplate}"/>
            </Style>

The fix for this issue will be available in the next LIB version (2018.3.1022) expected on Monday, October 22.

The fix will be available in the R1 2018 SP2 Release.

Available in LIB version 2017.3.1225, it will be also available in the R1 2018 Release.

workaround:
add the following visual state to the DocumentHostTemplate of the RadPaneGroup:

<VisualStateManager.VisualStateGroups>
    <VisualStateGroup x:Name="DropDownDisplayStates">
        <VisualState x:Name="DropDownButtonCollapsed" />
        <VisualState x:Name="DropDownButtonVisible">
            <Storyboard>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DropDownButtonElement" Storyboard.TargetProperty="Visibility">
                    <DiscreteObjectKeyFrame KeyTime="0">
                        <DiscreteObjectKeyFrame.Value>
                            <Visibility>Visible</Visibility>
                        </DiscreteObjectKeyFrame.Value>
                    </DiscreteObjectKeyFrame>
                </ObjectAnimationUsingKeyFrames>
            </Storyboard>
        </VisualState>
    </VisualStateGroup>
</VisualStateManager.VisualStateGroups>

and remove the trigger targeting the drop down button.

Available in LIB version: 2017.2.605

Available in LIB version 2016.3.1107, it will be also available in the 2017 R1 release.

Available in LIB version 2016.3.1107, it will be also available in the 2017 R1 release.

When the DropDownDisplayMode of the RadPaneGroup is set to Visible and that group is placed inside the DocumentHost calling the RemoveFromParent from a button places in the HeaderTemplate of the RadPane causes that RadPane instance to not be removed from the memory.

Available in LIB version 2016.3.1107, it will be also available in the 2017 R1 release.

Calling the LogicalTreeHelper.GetChildren() does not include the DocumentHost and its contents.
Available in LIB version 2015.3.1214, it will be also available in the 2016 Q1 Release.

Available in LIB version 2017.1.410 , it will be also available in the R2 2017 Release. 

This also causes the initial DataContext to be left out in the memory.
Available in LIB version 2015.3.1214, it will be also available in the 2016 Q1 Release.