This happens  whether there is available space in the active area are not. All items are rendered in the minimized area.

To work this around you can set the IsContentPreserved property after the control is loaded.


The fix for this issue will be available with the next LIB (version 2018.3.1224) expected on Monday, December 24.

The DisplayMemberPath property is not respected

Workaround:
Use an implicit style targeting RadOutlookBarItem instead of the DisplayMemberPath property. You can define the style in the xaml page (the view) where the RadOutlookBar control is defined.

<Window.Resources>
    <Style TargetType="telerik:RadOutlookBarItem">
        <Setter Property="Header" Value="{Binding ItemHeader}" />
    </Style>
</Window.Resources>

Collapse bar to a slim version, reorganizing the icons to be vertical without description but with hover over descriptions.

Approach to implement:

RadOutlookBar is minimizable by default.
Descriptions of the minimized RadOutlookBarItems can be achieved in WPF via:

<Style TargetType="telerik:RadOutlookBarItem" BasedOn="{StaticResource RadOutlookBarItemStyle}">
    <Style.Triggers>
        <DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type telerik:RadOutlookBar}}, Path=IsMinimized}" Value="True">
            <Setter Property="ToolTipService.ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=Header}" />
        </DataTrigger>
    </Style.Triggers>
</Style>

or by adding this trigger to the RadOutlookBarItem's ControlTemplate.Triggers

When the control is in minimized state and the items do not have an icon, the headers are not visible.

The foreground of the MinimizedButton and minimizeable arrow in Expression_Dark theme is wrong. The issue appears when the OutlookBar is minimized and the minimized button is clicked

You can work this around by extracting the RadOutlookBar style and slightly modify it. You can find the Grid panel that holds the title and the minimize button and position the elements in different columns. 
 
<!-- Other XAML from the template -->
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition />
        <ColumnDefinition />
    </Grid.ColumnDefinitions>
    <ContentControl x:Name="TitleElement" ContentTemplate="{TemplateBinding TitleTemplate}" Foreground="#FFDDDDDD"
                    FontWeight="Bold" HorizontalContentAlignment="Stretch" IsTabStop="False" Margin="5,3"
                    VerticalAlignment="Center" VerticalContentAlignment="Center"/>
    <telerik:RadToggleButton x:Name="MinimizeButton" Grid.Column="1"
                             IsChecked="{Binding IsMinimized, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                             Style="{TemplateBinding MinimizeButtonStyle}">
        <telerik:RadToggleButton.Visibility>
            <Binding Path="IsMinimizable" RelativeSource="{RelativeSource TemplatedParent}">
                <Binding.Converter>
                    <telerik:BooleanToVisibilityConverter/>
                </Binding.Converter>
            </Binding>
        </telerik:RadToggleButton.Visibility>
    </telerik:RadToggleButton>
</Grid>
<!-- Other XAML from the template -->

Another approach is to use the ChildrenOfTypeExtensions.ChildrenOfType<T>() method to get the Grid and the minimize button, and add set the required properties. Here is an example:

private void outlookBar_Loaded(object sender, RoutedEventArgs e)
{
    var minimizeButton = this.outlookBar.ChildrenOfType<RadToggleButton>().FirstOrDefault(x => x.Name == "MinimizeButton");
    if (minimizeButton != null)
    {
        var parentGrid = minimizeButton.ParentOfType<Grid>();
        parentGrid.ColumnDefinitions.Add(new ColumnDefinition());
        parentGrid.ColumnDefinitions.Add(new ColumnDefinition());
        Grid.SetColumn(minimizeButton, 1);
    }
}

 Clicking on MinimizedOutlookBarItem when there is Visibility set to Collapsed on one of the items, the RadOutlookBar selects different item.

Tab navigation is not working inside OutlookBar.

DECLINED: Duplicated with https://feedback.telerik.com/Project/143/Feedback/Details/113083-outlookbar-tab-navigation-is-not-working

As a workaround, you can add a custom Margin and Padding to the MinimizedOutlookBarItems:

<Style TargetType="outlookBarPrimitives:MinimizedOutlookBarItem">
    <Setter Property="Padding" Value="5" />
    <Setter Property="Margin" Value="2" />
</Style>

Where xmlns:outlookBarPrimitives="clr-namespace:Telerik.Windows.Controls.OutlookBar;assembly=Telerik.Windows.Controls.Navigation".

ItemTemplateSelector does not work. 

There is a workaround to extract the control template and edit the HeaderElement style with:
<ContentControl.ContentTemplateSelector>{TemplateBinding HeaderTemplateSelector}</ContentControl.ContentTemplateSelector>

The problem is fixed and will be available with Lib 2016.3.926.