How to make Search hint textbox background same as textbox background - wpf

Question

I have search hint textbox
<TextBox
TextChanged="textboxsearch_TextChanged"
Grid.Column="4" Margin="0,0,10,10" Height="22" >
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Triggers>
<Trigger Property="Text" Value="">
<Setter Property="Background" Value="{StaticResource SearchHint}" />
</Trigger>
<Trigger Property="IsKeyboardFocused" Value="True">
<Setter Property="Background" Value="White" />
</Trigger>
</Style.Triggers>
<Setter Property="VerticalAlignment" Value="Bottom"/>
</Style>
</TextBox.Style>
</TextBox>
here is SearchHint style
<VisualBrush x:Key="SearchHint" Stretch="None">
<VisualBrush.Visual>
<TextBox FontStyle="Italic" Background="White" Foreground="Gray" Text="Enter search text…" />
</VisualBrush.Visual>
</VisualBrush>
The search box back ground is filled by the searchhint style. The problem I have now is how can I make the width of the visual brush fill the size of the textbox. Right now it fills only a portion of the textbox.
The Text="Enter search text…" has a white background but the rest of the textbox is gray. I wanted to have a white background with gray hint text.

Answer 1

Give the TextBox in the VisualBrush a large padding (to the right), and give the VisualBrush a left alignment:
<VisualBrush x:Key="SearchHint" Stretch="None" AlignmentX="Left">
<VisualBrush.Visual>
<TextBox FontStyle="Italic" Background="White" Foreground="Gray" Text="Enter search text…"
Padding="0,0,1000,0" />
</VisualBrush.Visual>
</VisualBrush>

Answer 2

Well for what you're trying you can bind the Width of the control in the VisualBrush to your actual target's ActualWidth
something like:
<VisualBrush x:Key="SearchHint"
Stretch="None">
<VisualBrush.Visual>
<TextBox Background="White"
FontStyle="Italic"
Foreground="Gray"
Width="{Binding ElementName=tb, Path=ActualWidth}"
Height="{Binding ElementName=tb, Path=ActualHeight}"
Text="Enter search text…" />
</VisualBrush.Visual>
</VisualBrush>
...
<TextBox Grid.Column="4"
Height="22"
TextChanged="textboxsearch_TextChanged"
x:Name="tb"
Margin="0,0,10,10">
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Triggers>
<Trigger Property="Text"
Value="">
<Setter Property="Background"
Value="{StaticResource SearchHint}" />
</Trigger>
<Trigger Property="IsKeyboardFocused"
Value="True">
<Setter Property="Background"
Value="White" />
</Trigger>
</Style.Triggers>
<Setter Property="VerticalAlignment"
Value="Bottom" />
</Style>
</TextBox.Style>
</TextBox>
However
What you're "functionally" trying to do is referred to commonly as "Watermark" / "Placeholder text" and it's much simpler to use that approach than have a complicated VisualBrush with an actual control being turned into a Brush. Just my opinion.
If you want to try the Watermark approach This Answer gives a good example for you to work with.