Проблема в вашем root элементе. Замените StackPanel сеткой, и проблема решена.
Бонус: если вы новичок в WPF, я покажу вам, как реорганизовать ваши стили.
<Grid Background="Black">
<Grid.Resources>
<!-- Exemple of a style with a key to reuse -->
<Style x:Key="LeftButtonStyle" TargetType="Button">
<Setter Property="FontSize" Value="40" />
<Setter Property="Width" Value="80" />
<Setter Property="Height" Value="80" />
<Setter Property="Background" Value="#00000000" />
<Setter Property="Foreground" Value="White" />
<Setter Property="BorderBrush" Value="Transparent" />
</Style>
</Grid.Resources>
<!-- TitleBar buttons -->
<StackPanel
HorizontalAlignment="Right"
VerticalAlignment="Top"
Orientation="Horizontal">
<StackPanel.Resources>
<!-- Exemple of a implicit styles that will be set to each child with a matching type -->
<!-- Notice there is no x:Key -->
<Style TargetType="Button">
<Setter Property="Background" Value="Transparent" />
<Setter Property="Foreground" Value="White" />
<Setter Property="Padding" Value="8" />
<Setter Property="BorderThickness" Value="0" />
</Style>
</StackPanel.Resources>
<Button Content="b1" />
<Button Content="b2" />
<Button Content="b3" />
</StackPanel>
<!-- Menu Buttons -->
<StackPanel
HorizontalAlignment="Left"
VerticalAlignment="Center"
Orientation="Vertical">
<Button Content="icon" Style="{StaticResource LeftButtonStyle}" />
<Button Content="icon" Style="{StaticResource LeftButtonStyle}" />
<Button Content="icon" Style="{StaticResource LeftButtonStyle}" />
</StackPanel>
</Grid>
Обратите внимание, что нет ни RowDefinitions, ни ColumnDefinitions .
Вот как сделать то же самое с местом для установки содержимого (красная рамка для примера).
<Grid Background="Black">
<Grid.Resources>
<!-- Exemple of a style with a key to reuse -->
<Style x:Key="LeftButtonStyle" TargetType="Button">
<Setter Property="FontSize" Value="40" />
<Setter Property="Width" Value="80" />
<Setter Property="Height" Value="80" />
<Setter Property="Background" Value="#00000000" />
<Setter Property="Foreground" Value="White" />
<Setter Property="BorderBrush" Value="Transparent" />
</Style>
</Grid.Resources>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition />
</Grid.RowDefinitions>
<!-- TitleBar buttons -->
<StackPanel
Grid.Column="1"
HorizontalAlignment="Right"
Orientation="Horizontal">
<StackPanel.Resources>
<!-- Exemple of a implicit styles that will be set to each child with a matching type -->
<!-- Notice there is no x:Key -->
<Style TargetType="Button">
<Setter Property="Background" Value="Transparent" />
<Setter Property="Foreground" Value="White" />
<Setter Property="Padding" Value="8" />
<Setter Property="BorderThickness" Value="0" />
</Style>
</StackPanel.Resources>
<Button Content="b1" />
<Button Content="b2" />
<Button Content="b3" />
</StackPanel>
<!-- Menu Buttons -->
<StackPanel
Grid.RowSpan="2"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Orientation="Vertical">
<Button Content="icon" Style="{StaticResource LeftButtonStyle}" />
<Button Content="icon" Style="{StaticResource LeftButtonStyle}" />
<Button Content="icon" Style="{StaticResource LeftButtonStyle}" />
</StackPanel>
<Grid Background="Red" Grid.Row="1" Grid.Column="1"/>
</Grid>