Develop Reference

WPF C# ListBox IsMouseOver and IsSelected background

I am have made the following usercontrol, and all works well except the 2 Background setter properties with value transparent for both isMouseOver and IsSelected that do nothing..
<UserControl.Resources>
<DataTemplate x:Key="DeviceItemTemplate">
<Border Padding="10,5" Margin="10" BorderThickness="4" BorderBrush="Green" CornerRadius="20" Height="150" Width="150">
<StackPanel Orientation="Vertical">
<TextBox Name="screenNameTextBox"
Margin="10" Height="20"
Text="{Binding Name}" />
<TextBox
Margin="10" Height="20"
Text="{Binding Location}" />
</StackPanel>
</Border>
</DataTemplate>
<DataTemplate x:Key="DeviceItemTemplateSelected">
<Border Padding="10,5" Margin="10" BorderThickness="4" BorderBrush="Orange" CornerRadius="20" Height="150" Width="150" >
<StackPanel Orientation="Vertical" >
<TextBox Name="screenNameTextBox"
Margin="10" Height="20"
Text="{Binding Name}" />
<TextBox
Margin="10" Height="20"
Text="{Binding Location}" />
</StackPanel>
</Border>
</DataTemplate>
<Style TargetType="{x:Type ListBoxItem}" x:Key="DeviceContainerStyle">
<Setter Property="ContentTemplate" Value="{DynamicResource DeviceItemTemplate}"/>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True" >
<Setter Property="BorderThickness" Value="0" />
<Setter Property="Background" Value="Transparent" />
</Trigger>
<Trigger Property="IsSelected" Value="True">
<Setter Property="ContentTemplate" Value="{DynamicResource DeviceItemTemplateSelected}"/>
<Setter Property="BorderThickness" Value="0" />
<Setter Property="Background" Value="Transparent" />
</Trigger>
</Style.Triggers>
</Style>
</UserControl.Resources>
<Border
Grid.Column="0"
Margin="10"
BorderBrush="Silver"
BorderThickness="1">
<ListBox ItemsSource="{Binding Devices, UpdateSourceTrigger=PropertyChanged}"
SelectedItem="{Binding SelectedScreen, Mode=TwoWay}"
ItemContainerStyle="{StaticResource DeviceContainerStyle}"
ScrollViewer.HorizontalScrollBarVisibility="Disabled" >
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<WrapPanel IsItemsHost="True" Orientation="Horizontal" />
</ItemsPanelTemplate>
</ListBox.ItemsPanel>
</ListBox>
</Border>
This is how it looks
I also tried white, but got same results. I cannot get rid of this blue background.
What am I overlooking please?
-----After receiving mm8 answer-----
Placed all his code in a resource dictionary, made some changes to the solidcolorbrush and BorderThickness, and modified the style section from previous to:
<Style BasedOn="{StaticResource ListBoxItemSSDS}" TargetType="{x:Type ListBoxItem}" x:Key="DeviceContainerStyle">
<Setter Property="ContentTemplate" Value="{DynamicResource DeviceItemTemplate}"/>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="ContentTemplate" Value="{DynamicResource DeviceItemTemplateSelected}"/>
</Trigger>
</Style.Triggers>
</Style>

You need to define a custom ControlTemplate for the ListBoxItem to be able to the background of it when the IsMouseOver and IsSelected properties are set.
You can right-click on a ListBoxItem in design mode in Visual Studio or in Blend and choose Edit Template->Edit a Copy to copy the default template into your XAML markup and then edit it as per your requirements.
Here is what it looks like and the resources that are involved:
<Style x:Key="FocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="2" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<SolidColorBrush x:Key="Item.MouseOver.Background" Color="#1F26A0DA"/>
<SolidColorBrush x:Key="Item.MouseOver.Border" Color="#a826A0Da"/>
<SolidColorBrush x:Key="Item.SelectedInactive.Background" Color="#3DDADADA"/>
<SolidColorBrush x:Key="Item.SelectedInactive.Border" Color="#FFDADADA"/>
<SolidColorBrush x:Key="Item.SelectedActive.Background" Color="#3D26A0DA"/>
<SolidColorBrush x:Key="Item.SelectedActive.Border" Color="#FF26A0DA"/>
<Style x:Key="ListBoxItemStyle1" TargetType="{x:Type ListBoxItem}">
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="Padding" Value="4,1"/>
<Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ListBoxItem}">
<Border x:Name="Bd" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
<ControlTemplate.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsMouseOver" Value="True"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="Bd" Value="{StaticResource Item.MouseOver.Background}"/>
<Setter Property="BorderBrush" TargetName="Bd" Value="{StaticResource Item.MouseOver.Border}"/>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="Selector.IsSelectionActive" Value="False"/>
<Condition Property="IsSelected" Value="True"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="Bd" Value="{StaticResource Item.SelectedInactive.Background}"/>
<Setter Property="BorderBrush" TargetName="Bd" Value="{StaticResource Item.SelectedInactive.Border}"/>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="Selector.IsSelectionActive" Value="True"/>
<Condition Property="IsSelected" Value="True"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="Bd" Value="{StaticResource Item.SelectedActive.Background}"/>
<Setter Property="BorderBrush" TargetName="Bd" Value="{StaticResource Item.SelectedActive.Border}"/>
</MultiTrigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
You may for example change the Color of the Item.MouseOver.Background and Item.SelectedActive.Background resources.

WPF : How to change the MouseOver/Selected background colour of a GridView row

I'm having a hard time finding a solution to this so would appreciate any help.
I need to be able to change the background of a GridView row when the mouse is over or of if the row is selected.
Here's my ListView Style.
<Style x:Key="{x:Static GridView.GridViewScrollViewerStyleKey}"
TargetType="ScrollViewer">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ScrollViewer">
<Grid Background="{TemplateBinding Background}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<DockPanel Margin="{TemplateBinding Padding}">
<ScrollViewer DockPanel.Dock="Top"
HorizontalScrollBarVisibility="Hidden"
VerticalScrollBarVisibility="Hidden"
Focusable="false">
<GridViewHeaderRowPresenter Margin="2,0,2,0"
Columns="{Binding Path=TemplatedParent.View.Columns,
RelativeSource={RelativeSource TemplatedParent}}"
ColumnHeaderContainerStyle="{Binding
Path=TemplatedParent.View.ColumnHeaderContainerStyle,
RelativeSource={RelativeSource TemplatedParent}}"
ColumnHeaderTemplate="{Binding
Path=TemplatedParent.View.ColumnHeaderTemplate,
RelativeSource={RelativeSource TemplatedParent}}"
ColumnHeaderTemplateSelector="{Binding
Path=TemplatedParent.View.ColumnHeaderTemplateSelector,
RelativeSource={RelativeSource TemplatedParent}}"
AllowsColumnReorder="{Binding
Path=TemplatedParent.View.AllowsColumnReorder,
RelativeSource={RelativeSource TemplatedParent}}"
ColumnHeaderContextMenu="{Binding
Path=TemplatedParent.View.ColumnHeaderContextMenu,
RelativeSource={RelativeSource TemplatedParent}}"
ColumnHeaderToolTip="{Binding
Path=TemplatedParent.View.ColumnHeaderToolTip,
RelativeSource={RelativeSource TemplatedParent}}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</ScrollViewer>
<ScrollContentPresenter Name="PART_ScrollContentPresenter"
KeyboardNavigation.DirectionalNavigation="Local"/>
</DockPanel>
<ScrollBar Name="PART_HorizontalScrollBar"
Orientation="Horizontal"
Grid.Row="1"
Maximum="{TemplateBinding ScrollableWidth}"
ViewportSize="{TemplateBinding ViewportWidth}"
Value="{TemplateBinding HorizontalOffset}"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"/>
<ScrollBar Name="PART_VerticalScrollBar"
Grid.Column="1"
Maximum="{TemplateBinding ScrollableHeight}"
ViewportSize="{TemplateBinding ViewportHeight}"
Value="{TemplateBinding VerticalOffset}"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="GridViewColumnHeaderGripper" TargetType="Thumb">
<Setter Property="Width" Value="18"/>
<Setter Property="Background" Value="{StaticResource NormalBorderBrush}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Border Padding="{TemplateBinding Padding}"
Background="Transparent">
<Rectangle HorizontalAlignment="Center"
Width="1"
Fill="{TemplateBinding Background}"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="{x:Type GridViewColumnHeader}"
TargetType="GridViewColumnHeader">
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Foreground"
Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="GridViewColumnHeader">
<Grid>
<Border Name="HeaderBorder"
BorderThickness="0,1,0,1"
BorderBrush="{StaticResource NormalBorderBrush}"
Background="{StaticResource ControlDarkBrush}"
Padding="2,0,2,0">
<ContentPresenter Name="HeaderContent"
Margin="0,0,0,1"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
RecognizesAccessKey="True"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Border>
<Thumb x:Name="PART_HeaderGripper"
HorizontalAlignment="Right"
Margin="0,0,-9,0"
Style="{StaticResource GridViewColumnHeaderGripper}"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="true">
<Setter TargetName="HeaderBorder"
Property="Background" Value="{StaticResource NormalBrush}"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter TargetName="HeaderBorder"
Property="Background" Value="{StaticResource PressedBrush}"/>
<Setter TargetName="HeaderContent"
Property="Margin" Value="1,1,0,0"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground"
Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="Role" Value="Floating">
<Setter Property="Opacity" Value="0.7"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="GridViewColumnHeader">
<Canvas Name="PART_FloatingHeaderCanvas">
<Rectangle Fill="#60000000"
Width="{TemplateBinding ActualWidth}"
Height="{TemplateBinding ActualHeight}"/>
</Canvas>
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="Role" Value="Padding">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="GridViewColumnHeader">
<Border Name="HeaderBorder"
BorderThickness="0,1,0,1"
BorderBrush="{StaticResource NormalBorderBrush}"
Background="{StaticResource ControlDarkBrush}"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
<Style x:Key="StyleListView" TargetType="ListView">
<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="VerticalContentAlignment" Value="Center"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListView">
<Border Name="Border"
BorderThickness="1"
BorderBrush="{StaticResource BorderLightBrush}"
Background="{StaticResource ControlDarkBrush}">
<ScrollViewer Style="{DynamicResource
{x:Static GridView.GridViewScrollViewerStyleKey}}">
<ItemsPresenter />
</ScrollViewer>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsGrouping"
Value="true">
<Setter Property="ScrollViewer.CanContentScroll"
Value="false"/>
</Trigger>
<Trigger Property="IsEnabled"
Value="false">
<Setter TargetName="Border"
Property="Background"
Value="{StaticResource DisabledBorderDarkBrush}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
The closest I've come to getting to an answer is this post (code is omitted from my Style as it didn't have any effect) :-
Change ListViewItem background colour on mouse over
However, I'm still unable to control the mouseover and selected colours. I'm getting closer as the post above suggests that it is a button which is actually doing the highlighting and selecting but I don't know how to incorporate that in to my existing style.
Here's the current effect of the Style. The top row is selected, the third row is mouse-overed, the other two rows are normal.
Mouseover / selected colours
BTW, the Style I'm using was extracted from Blend and I tweaked it to use my app colours.
I don't think my Class code would be relevant here but correct me if I'm wrong and I will post it.

I think you need to add a style for the ListViewItems and set the ListView.ItemContainerStyle:
<!-- New style -->
<Style x:Key="StyleListViewItem" TargetType="ListViewItem">
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Green"/>
</Trigger>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Background" Value="Blue"/>
</Trigger>
</Style.Triggers>
</Style>
<!-- Existing style -->
<Style x:Key="StyleListView" TargetType="ListView">
<Setter Property="ItemContainerStyle" Value="{StaticResource StyleListViewItem}"/>
<Setter Property="SnapsToDevicePixels" Value="true"/>
<!-- ... -->
</Style>

Access Element in ControlTemplate from Style

I want to Style an Border inside a ControlTemplate. But I don't know how to access it. My Style looks like this:
<Style x:Key="RedCell" TargetType="DataGridCell" BasedOn="{StaticResource MYDGCellStyle}">
<Setter Property="Foreground" Value="White" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="DataGridCell">
<Grid>
<Grid Grid.ZIndex="86" x:Name="CellContenGrind" Background="{TemplateBinding Background}" />
<Border Grid.ZIndex="87" x:Name="ContentBorder" BorderBrush="White" Background="Crimson" CornerRadius="25">
<ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" HorizontalAlignment="Center" VerticalAlignment="Center" />
</Border>
<Grid Grid.ZIndex="88" x:Name="CellGridFocused"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsFocused" Value="True">
<Setter Property="Background" TargetName="CellGridFocused" Value="{DynamicResource Brush_DataGridCellFocused}" />
</Trigger>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Background" TargetName="CellContenGrind" Value="{DynamicResource Brush_DataGridSelected}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<DataTrigger Binding="{Binding ungelesen}" Value="0">
<Setter Property="Background" Value="Green"/>
</DataTrigger>
</Style.Triggers>
</Style>
I need the Background="{TemplateBinding Background}" on my CellContentGrind because of some other stuff, so I can't just move that to my Border.
If ungelesen = 0, I want the Background of my ContentBorder to be Green. How can I do that?

Why not move the Style to the Border itself?
<Border Grid.ZIndex="87"x:Name="ContentBorder" BorderBrush="White" CornerRadius="25">
<Border.Style>
<Style TargetType="Border">
<Setter Property="Background" Value="Crimson" />
<Style.Triggers>
<DataTrigger Binding="{Binding ungelesen}" Value="0">
<Setter Property="Background" Value="Green"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Border.Style>
<ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" HorizontalAlignment="Center" VerticalAlignment="Center" />
</Border>
Note that Background is set with Setter now to make overridable with DataTrigger.

How to set the mouse over trigger in custom MenuItem Control of WPF?

I have a MenuItem style:
<Style TargetType="{x:Type MenuItem}" BasedOn="{StaticResource {x:Type MenuItem}}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}"
Height="22" SnapsToDevicePixels="true">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="4"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="menuHeaderContainer" Grid.Column="0"
ContentSource="Header" HorizontalAlignment="Left"
Margin="{TemplateBinding Padding}" RecognizesAccessKey="True"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
VerticalAlignment="Center"/>
<TextBlock x:Name="menuGestureText" Grid.Column="2"
Margin="{TemplateBinding Padding}" Opacity="0.7" Text="{TemplateBinding InputGestureText}"
VerticalAlignment="Center"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="Black"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="Red"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="Black"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
My Control is:
<Grid>
<Menu VerticalAlignment="Center" HorizontalAlignment="Center">
<MenuItem Header="Header">
<MenuItem Header="Second Level"/>
</MenuItem>
</Menu>
I am willing to set a new style when mouse over the menuitem, the background will be black. My style works but only the second level menuitem.
When mouse over the first level menuitem, it still the default style:
When mouse over the second level menuitem, my style works(it will be black when mouse over it):
Why my style can not work at first level MenuItem?

Control Template for Top Level MenuItem and Sub-MenuItems are different. Top level MenuItem also needs to define how the SubMenu Items will Popup.
You can write multiple ControlTemplate for each type of Menu Item. And then use following Style to Set the MenuItem styles:
<Style x:Key="{x:Type MenuItem}" TargetType="MenuItem">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Style.Triggers>
<Trigger Property="Role" Value="TopLevelHeader">
<Setter Property="Template" Value="{StaticResource {x:Static MenuItem.TopLevelHeaderTemplateKey}}"/>
<Setter Property="Grid.IsSharedSizeScope" Value="true"/>
</Trigger>
<Trigger Property="Role" Value="TopLevelItem">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.TopLevelItemTemplateKey}}"/>
</Trigger>
<Trigger Property="Role" Value="SubmenuHeader">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.SubmenuHeaderTemplateKey}}"/>
</Trigger>
<Trigger Property="Role" Value="SubmenuItem">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.SubmenuItemTemplateKey}}"/>
</Trigger>
</Style.Triggers>
</Style>
For more detail see this post

How can I make the background color of a node in a TreeView goes all the way out to the right edge?

How can I make the background color of a node in a TreeView goes all the way out to the right edge?
Background
I have a TreeView where all nodes that have children are gray. Leaf nodes have an icon with descriptive text.

If i understand the problem right you are looking to stretch the treeview items.
If that is the case check Here

It's an easy fix. Unfortunately it'll require pretty much code since you'll have to re-template the TreeViewItem. The TreeViewItem's template looks like this
<ControlTemplate TargetType="{x:Type TreeViewItem}">
<Grid>
<Grid.ColumnDefinitions>
<!-- The Expand/Collapse ToggleButton -->
<ColumnDefinition MinWidth="19" Width="Auto"/>
<!-- The TreeViewItem -->
<ColumnDefinition Width="Auto"/>
<!-- The Children -->
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<ToggleButton x:Name="Expander" ClickMode="Press" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ExpandCollapseToggleStyle}"/>
<Border x:Name="Bd" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Column="1" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
<ContentPresenter x:Name="PART_Header" ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Border>
<ItemsPresenter x:Name="ItemsHost" Grid.ColumnSpan="2" Grid.Column="1" Grid.Row="1"/>
</Grid>
As you can see, the TreeViewItem itself is in Column 1 which has Width set to Auto while the children (ItemsPresenter) is in Grid.Column 1 and 2 and the Column 2 has Width set to *. To get the TreeViewItem itself to get the same Width as its children we can remove the middle Column (Column 1) and just use Grid.Column="1" for both.
Here's the default Template for TreeViewItem with this "Horizontal Stretch" fix. You'll have to add a reference to PresentationFramework.Aero
<Style x:Key="TreeViewItemFocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<PathGeometry x:Key="TreeArrow" Figures="M0,0 L0,6 L6,0 z"/>
<Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type ToggleButton}">
<Setter Property="Focusable" Value="False"/>
<Setter Property="Width" Value="16"/>
<Setter Property="Height" Value="16"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Background="Transparent" Height="16" Padding="5,5,5,5" Width="16">
<Path x:Name="ExpandPath" Data="{StaticResource TreeArrow}" Fill="Transparent" Stroke="#FF989898">
<Path.RenderTransform>
<RotateTransform Angle="135" CenterY="3" CenterX="3"/>
</Path.RenderTransform>
</Path>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Stroke" TargetName="ExpandPath" Value="#FF1BBBFA"/>
<Setter Property="Fill" TargetName="ExpandPath" Value="Transparent"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="RenderTransform" TargetName="ExpandPath">
<Setter.Value>
<RotateTransform Angle="180" CenterY="3" CenterX="3"/>
</Setter.Value>
</Setter>
<Setter Property="Fill" TargetName="ExpandPath" Value="#FF595959"/>
<Setter Property="Stroke" TargetName="ExpandPath" Value="#FF262626"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="StretchTreeViewItemStyle"
TargetType="{x:Type TreeViewItem}"
xmlns:Microsoft_Windows_Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
<Setter Property="Padding" Value="1,0,0,0"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="FocusVisualStyle" Value="{StaticResource TreeViewItemFocusVisual}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TreeViewItem}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="19" Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<ToggleButton x:Name="Expander" ClickMode="Press" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ExpandCollapseToggleStyle}"/>
<Border x:Name="Bd" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Column="1" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
<ContentPresenter x:Name="PART_Header" ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Border>
<ItemsPresenter x:Name="ItemsHost" Grid.Column="1" Grid.Row="1"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="false">
<Setter Property="Visibility" TargetName="ItemsHost" Value="Collapsed"/>
</Trigger>
<Trigger Property="HasItems" Value="false">
<Setter Property="Visibility" TargetName="Expander" Value="Hidden"/>
</Trigger>
<Trigger Property="IsSelected" Value="true">
<Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsSelected" Value="true"/>
<Condition Property="IsSelectionActive" Value="false"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
</MultiTrigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="VirtualizingStackPanel.IsVirtualizing" Value="true">
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<VirtualizingStackPanel/>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
And then you can use it like this
<TreeView ...>
<TreeView.ItemContainerStyle>
<Style TargetType="{x:Type TreeViewItem}" BasedOn="{StaticResource StretchTreeViewItemStyle}">
<!-- Add your own setters if applicable -->
</Style>
</TreeView.ItemContainerStyle>
</TreeView>