Why is the caret in this editable ComboBox exceeding the bounds?

Question

I'm trying to build a template for combobox based on msdn examples but I have strange error. More precisely if I wrote long text in normal combobox the text is moving to left and we see caret all the time. However in msdn examples when I wrote long text caret is moving outside of combo. Here is picute how it look like:

When I select text it looks like this:

I have used examples from Framework 3.5 and Framework 4 combobox help and after copy and paste I'm keep getting the same situation. Thanks for any help.

EDIT:

    <Window x:Class="ComboBoxTest.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="Framework 3.5" Height="350" Width="525">

    <Window.Resources>
        <ControlTemplate x:Key="ComboBoxToggleButton" TargetType="ToggleButton">
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition Width="20" />
                </Grid.ColumnDefinitions>
                <Border x:Name="Border" Grid.ColumnSpan="2" CornerRadius="2" Background="White"
                        BorderBrush="Black" BorderThickness="1" />
                <Border Grid.Column="0" CornerRadius="2,0,0,2" Margin="1" Background="#FFF"
                        BorderBrush="Black" BorderThickness="0,0,1,0" />
                <Path x:Name="Arrow" Grid.Column="1" Fill="#444" HorizontalAlignment="Center"
                      VerticalAlignment="Center" Data="M 0 0 L 4 4 L 8 0 Z" />
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger Property="ToggleButton.IsMouseOver" Value="true">
                    <Setter TargetName="Border" Property="Background" Value="Black" />
                </Trigger>
                <Trigger Property="ToggleButton.IsChecked" Value="true">
                    <Setter TargetName="Border" Property="Background" Value="LightGray" />
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter TargetName="Border" Property="Background" Value="#EEE" />
                    <Setter TargetName="Border" Property="BorderBrush" Value="#AAA" />
                    <Setter Property="Foreground" Value="#888" />
                    <Setter TargetName="Arrow" Property="Fill" Value="#888" />
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>

        <!-- something is wrong here but what?? -->
        <ControlTemplate x:Key="ComboBoxTextBox" TargetType="TextBox">
            <Border x:Name="PART_ContentHost" Focusable="False" />
        </ControlTemplate>

        <Style x:Key="MsComboBox" TargetType="ComboBox">
            <Setter Property="SnapsToDevicePixels" Value="true" />
            <Setter Property="OverridesDefaultStyle" Value="true" />
            <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
            <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
            <Setter Property="ScrollViewer.CanContentScroll" Value="true" />
            <Setter Property="MinWidth" Value="120" />
            <Setter Property="MinHeight" Value="20" />
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="ComboBox">
                        <Grid>
                            <ToggleButton Name="ToggleButton" Template="{StaticResource ComboBoxToggleButton}" Grid.Column="2"
                                          Focusable="false" IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
                                          ClickMode="Press"></ToggleButton>

                            <ContentPresenter Name="ContentSite" IsHitTestVisible="False" Content="{TemplateBinding SelectionBoxItem}"
                                              ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                              ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Margin="3,3,23,3" VerticalAlignment="Center"
                                              HorizontalAlignment="Left" />

                            <TextBox x:Name="PART_EditableTextBox" 
                                    Style="{x:Null}" 
                                    Template="{StaticResource ComboBoxTextBox}"
                                    HorizontalAlignment="Left" VerticalAlignment="Center" Margin="3,3,23,3" Focusable="True" Background="Transparent"
                                    Visibility="Hidden" IsReadOnly="{TemplateBinding IsReadOnly}" />

                            <Popup Name="Popup" Placement="Bottom" IsOpen="{TemplateBinding IsDropDownOpen}" AllowsTransparency="True"
                                   Focusable="False" PopupAnimation="Slide">
                                <Grid Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding ActualWidth}"
                                      MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                    <Border x:Name="DropDownBorder" Background="Black" BorderThickness="1"
                                            BorderBrush="#888" />
                                    <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
                                        <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
                                    </ScrollViewer>
                                </Grid>
                            </Popup>
                        </Grid>
                        <ControlTemplate.Triggers>                              
                            <Trigger Property="IsEditable" Value="true">
                                <Setter Property="IsTabStop" Value="false" />
                                <Setter TargetName="PART_EditableTextBox" Property="Visibility" Value="Visible" />
                                <Setter TargetName="ContentSite" Property="Visibility" Value="Hidden" />
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>

    <Grid>
        <ComboBox Height="23" HorizontalAlignment="Left" Margin="12,12,0,0" Name="comboBox1" VerticalAlignment="Top" IsEditable="True"
                  Width="185" Style="{StaticResource MsComboBox}" />
        <ComboBox Height="23" HorizontalAlignment="Left" Margin="12,41,0,0" Name="comboBox2" VerticalAlignment="Top" IsEditable="True" Width="185" />

    </Grid>
</Window>

Answer 1

Your Border doesn't contain functionality for automatically scrolling the content when it goes off the screen, so it just keeps drawing it. Change it to a ScrollViewer and it will work.

<ControlTemplate x:Key="ComboBoxTextBox" TargetType="TextBox">
    <ScrollViewer x:Name="PART_ContentHost" Focusable="False" />
</ControlTemplate>