Remove Extra Space around Listbox
This is the default control template for ListBox:
<ControlTemplate x:Key="ListBoxControlTemplate1" TargetType="{x:Type ListBox}">
<Border x:Name="Bd" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="1" SnapsToDevicePixels="True">
<ScrollViewer Focusable="False" Padding="{TemplateBinding Padding}">
<ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</ScrollViewer>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
</Trigger>
<Trigger Property="IsGrouping" Value="True">
<Setter Property="ScrollViewer.CanContentScroll" Value="False"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
Notice the Padding="1" on Border named Bd. Since this is harcoded and not template bound, you can either retemplate the ListBox and set the padding to 0, or since Padding on the ScollViewer has a TemplateBinding to the Padding of the ListBox, you can set the Padding on your ListBox to -1 to offset the padding on the border.
The control template of a ListBox looks like this:
<ControlTemplate TargetType="{x:Type ListBox}">
<Border Name="Bd"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
SnapsToDevicePixels="true"
Padding="1"> <!-- This might be the problem -->
<!-- ... -->