question

You could set the button in the window to make the UserControl full screen. Please try to refer to the following code.
The code of UserControl:

 <UserControl x:Class="WpfApp1.UserControl1"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
              xmlns:local="clr-namespace:WpfApp1"
              mc:Ignorable="d" 
              d:DesignHeight="450" d:DesignWidth="800">
     <Border BorderThickness="2" BorderBrush="Blue">
         <Grid Margin="5">
             <Grid.RowDefinitions>
                 <RowDefinition Height="Auto"/>
                 <RowDefinition Height="*"/>
             </Grid.RowDefinitions>
             <Grid>
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition x:Name="colIndex" Width="2*"/>
                     <ColumnDefinition x:Name="colContent" Width="*"/>
                 </Grid.ColumnDefinitions>
                 <ToolBarTray Background="Transparent" HorizontalAlignment="Stretch" Grid.ColumnSpan="2">
                     <ToolBar Background="Transparent" HorizontalAlignment="Stretch">
                         <Button Content="Afficher/Masquer liste des pieux" Margin="10,0,10,0"></Button>
                         <Button Grid.Column="1" x:Name="ResetPovButton" Content="Reinitialiser point de vue"  Click="ResetPovButton_Click"  Margin="10,0,10,0"/>
                         <Button Content="Activer/Desactiver rotation" Margin="10,0,10,0" x:Name="RotationButton" ></Button>
                         <Button Content="Exporter"  Margin="10,0,10,0" x:Name="ExportButton"></Button>
                         <Button Content="Mode Plein Ecran"  Margin="10,0,10,0" x:Name="FullScreenButton" Click="FullScreenButton_Click"></Button>
                     </ToolBar>
                 </ToolBarTray>
             </Grid>
             <WindowsFormsHost Grid.Row="1" Name="WinFormsHost"/>
         </Grid>
     </Border>
        
 </UserControl>

Code for Windows xaml：

 <Window x:Class="WpfApp1.MainWindow"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:local="clr-namespace:WpfApp1" x:Name="Window_Main"
         mc:Ignorable="d"
         Title="MainWindow" Height="450" Width="800">
     <Grid x:Name="grdWrapper">
         <Grid.ColumnDefinitions>
             <ColumnDefinition x:Name="colIndex" Width="150"></ColumnDefinition>
             <ColumnDefinition x:Name="colContent" Width="*"></ColumnDefinition>
         </Grid.ColumnDefinitions>
         <Grid.RowDefinitions>
             <RowDefinition x:Name="rowHeader" Height="70"></RowDefinition>
             <RowDefinition x:Name="rowContent" Height="*"></RowDefinition>
             <RowDefinition x:Name="rowFooter" Height="50"></RowDefinition>
         </Grid.RowDefinitions>
         <local:UserControl1 x:Name="uc_Index" Grid.Column="1" Grid.Row="1"></local:UserControl1>
         <ToggleButton x:Name="btnFullScreen" Grid.Column="1" Grid.Row="1" HorizontalAlignment="Right" 
                       VerticalAlignment="Top" Margin="0,0,10,0" Checked="btnFullScreen_Checked_1" 
                       Unchecked="btnFullScreen_Unchecked_1" Content="Full Screen"></ToggleButton>
     </Grid>
 </Window>

Code for Windows xaml.cs：

  public partial class MainWindow : Window
       {
         public MainWindow()
         {
           InitializeComponent();
         }
         private void btnFullScreen_Checked_1(object sender, RoutedEventArgs e)
         {
           colIndex.MaxWidth = 1;
           rowHeader.MaxHeight = 1;
           rowFooter.MaxHeight = 1;
        
           btnFullScreen.Content = "Exit Full Screen";
        
           Window_Main.WindowStyle = WindowStyle.None;
           Window_Main.WindowState = WindowState.Maximized;
         }
         private void btnFullScreen_Unchecked_1(object sender, RoutedEventArgs e)
         {
           colIndex.MaxWidth = 100;
           rowHeader.MaxHeight = 100;
           rowFooter.MaxHeight = 100;
        
           btnFullScreen.Content = "Full Screen";
        
           Window_Main.WindowStyle = WindowStyle.ThreeDBorderWindow;
           Window_Main.WindowState = WindowState.Maximized;
        
         }
       }

The picture of result:

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@HuiLiu-MSFT thanks for the answer but this is not the solution am searching for .

I want to let the full-screen button in the toolbar (inside the user control) not in the window