The IsMinimizedChanged method does not inspect the ACTUAL width of the control

OutlookBar: Visual state of selected items in not correct

The ActiveItemsCount is not working

The selection of a minimized item cannot be visually cancelled.

When the ItemsSource of the control is populated from a background thread, users are not allowed to manually set the position of the horizontal splitter.

Content is lost when setting IsMinimizable to false and the OutlookBar is minimized

Add the posibilty for the RadOutlookBar control to have Width less than the specified MinWidth, when Minimized.

TabIndex/Tab navigation is not working inside OutlookBar.

The foreground of the content in the pop-up when the OutlookBar is minimized in Windows8 (Metro) and Windows8Touch themes is wrong - it is white like the background.

Tooltip is shown on horizontal splitter when ActiveItemsMaxCount = 0 but it shouldn't show since the splitter cannot be moved

When the selected item is Collapsed, the RadOutlookBar still displays its content and Title

When the OutlookBar is minimized and you click the right splitter without moving it. Then the outlookbar is still minimized does not expand to the previous expanded position

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>

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

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);
    }
}