Possible work-around of this problem:

private void RadDocking_ElementLoaded ( object sender, LayoutSerializationEventArgs e )
 {
     if ( e.AffectedElement is RadPaneGroup )
     {
         var PaneGroup = e.AffectedElement as RadPaneGroup;
         var Panes = PaneGroup.EnumeratePanes().ToList();
         var SelectedIndex = PaneGroup.SelectedIndex;

         for ( int Index = 0; Index < Panes.Count; Index++ )
         {
             if ( ( Panes[ Index ].IsHidden || !Panes[ Index ].IsPinned ) && Index <= SelectedIndex )
             {
                 SelectedIndex++;
             }
         }

         PaneGroup.SelectedIndex = SelectedIndex;
     }
 }

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.

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.

Having Problems with unreadable Tabs in RadPaneGroup: TabsSqueeze.png

I think I found a Solution with OverflowMode and ScrollViewer:

<telerik:RadPaneGroup x:Name="DocumentGroup" ScrollViewer.HorizontalScrollBarVisibility="Auto" OverflowMode="Scroll">

Problem:

After Loading Layout the OverflowMode is not respected anymore.

I found this ticket in Silverlight, what discribes the Problem and is approved: Docking: I would like OverflowMode to work for all RadPaneGroup in xaml/codebehind and this be persisted through SaveLayout/LoadLayout

And i found this documentation, which say, using OverflowMode can lead to unexpected behaviors:
https://docs.telerik.com/devtools/wpf/controls/raddocking/general-information/not-supported-properties

so i report this as a feature instead of a bug.

Please sea atteched an example project: TabWrapProblem.zip
Usage: Save and than Load the Layout -> Scrolling of tabs break

Just like in Visual Studio, a floating pane, which I want to dock (for example to the bottom), can either take the whole width or just the width that is left.

For example:
At start, the panes in the bottom does not consume the whole window width. After undocking the panes and docking them back, the compass only shows me an option to dock the panes while consuming the whole width.

I have no option to return to the original state (like in Start.png). The only way to return to that state is undock and re-dock the right pane, and then dock and re-dock the left pane.

Visual Studio has this feature (see image).

Provide an easy functionality to exclude a pane or group of panes from reordering when AllowDragReorder is set to True.

Currently the arrow buttons appear only when the TabStripPlacement is set to top.

Expose information about the used Compass when docking a Pane.

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

When the Dock's auto-hide area is is about to be shown on two monitors simultaneously it is shown only on one of the monitors.

After upgrading to Telerik WPF 2019.1.116 from 2018.3.1010 we receive a lot of:

System.ComponentModel.Win32Exception (0x80004005): Invalid window handle
   MS.Win32.UnsafeNativeMethods.GetWindowText(HandleRef hWnd, StringBuilder lpString, Int32 nMaxCount)

Exceptions. It makes out application very slow.

In version 2018 everyting was OK. See attached scerenshots.

The RadDocking control template hosts several AutoHideArea elements, one for each auto-hide placement - left, top, right, bottom. When you unpin a RadPane, it goes into one of those areas. The active auto-hide area where the pane will go is determined by the State of the topmost RadSplitContainer.

Allow manually setting the auto-hide area where the RadPane will go when unpinned.

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 RadPaneGroup inherits RadTabControl and the TabStripPlacement property. Currently the RadPaneGroup does not support setting this property as the corresponding ControlTemplates are not implemented.