question

Below is an example about Balloon tooltip above the overlay, you could try to refer to it. For the content of the tooltip position, you can refer to here.
MainWindow.xaml:

 <Window x:Class="tooltipballoon.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:tooltipballoon"
         mc:Ignorable="d"  Name="window" Loaded="window_Loaded"
         Title="MainWindow" Height="450" Width="800">
     <Window.ToolTip>
         <ToolTip x:Name="tt">Press G to Surface or Hide</ToolTip>
     </Window.ToolTip>
     <Window.Resources>
         <Style x:Key="{x:Type ToolTip}" TargetType="ToolTip">
             <Setter Property="Placement" Value="Top"/>
             <Setter Property="PlacementTarget" Value="{Binding ElementName=winodw}"/>
             <Setter Property="PlacementRectangle" Value="10,0,700,0"/>
             <Setter Property="OverridesDefaultStyle" Value="true" />
             <Setter Property="HorizontalOffset" Value="250" />
             <Setter Property="VerticalOffset" Value="15" />
             <Setter Property="Background" Value="LightSkyBlue" />
             <Setter Property="Foreground" Value="White" />
             <Setter Property="Template">
                 <Setter.Value>
                     <ControlTemplate TargetType="ToolTip">
                         <Canvas Width="200" Height="100">
                             <Path x:Name="Container"
                   Canvas.Left="0" Canvas.Top="0"                                 
                   Margin="0" Data="M 0,45 L10,40  L160,40 C160,40 170,40 170,35 L170 10 C170,10 170,5 160,5 L10,5 C10,5 0,5 0,10 L0,35 C0,35 0,40 0,45 Z"                                    
                   Stroke="LightSkyBlue">
                                 <Path.Fill>
                                     <SolidColorBrush Color="DeepSkyBlue"/>
                                 </Path.Fill>
                             </Path>
                             <TextBlock Canvas.Left="15" Canvas.Top="15"  Width="200"
                        Height="55" Text="{TemplateBinding Content}" TextWrapping="Wrapwithoverflow" />
                         </Canvas>
                     </ControlTemplate>
                 </Setter.Value>
             </Setter>
         </Style>
     </Window.Resources>
     <Grid>
     </Grid>
 </Window>

MainWindow.xaml.cs:

 using System.Windows;
 using System.Windows.Input;
    
 namespace tooltipballoon
 {
   public partial class MainWindow : Window
   {
     public MainWindow()
     {
       InitializeComponent();
     }
     private void window_Loaded(object sender, RoutedEventArgs e)
     {
       var window = Window.GetWindow(this);
       window.KeyDown += windowKeyDown;
     }
     private void windowKeyDown(object sender, KeyEventArgs e)
     {
       var window =sender as Window;
          
       tt.PlacementTarget=window;
       if (e.Key == Key.G)
       {
         if (tt.IsOpen == false)
         {
           tt.IsOpen = true;
         }
         else
         {
           tt.IsOpen = false;
         }
       }
     }
   }
 }

The result:

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