<Window xmlns="https://github.com/avaloniaui"
        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"
        Width="210" Height="170"
        x:Class="AvaloniaUI.TransparentWithShapes"
        Title="TransparentWithShapes"
        SystemDecorations="None"
        TransparencyLevelHint="Transparent"
        Background="Transparent">

    <Grid x:Name="root">
        <!-- 这里的 Path 既画边框/填充，也会用于 Clip  -->
        <Path x:Name="shapePath"
              Stroke="DarkGray"
              StrokeThickness="1"
              Stretch="None">

            <Path.Fill>
                <LinearGradientBrush StartPoint="20%,0%" EndPoint="80%,100%">
                    <GradientStop Color="White" Offset="0"/>
                    <GradientStop Color="White" Offset="0.45"/>
                    <GradientStop Color="LightBlue" Offset="0.9"/>
                    <GradientStop Color="Gray" Offset="1"/>
                </LinearGradientBrush>
            </Path.Fill>

            <Path.Data>
                <PathGeometry>
                    <PathFigure StartPoint="20,0" IsClosed="True">
                        <LineSegment Point="140,0"/>
                        <ArcSegment Point="160,20" Size="20,20" SweepDirection="Clockwise"/>
                        <LineSegment Point="160,60"/>
                        <ArcSegment Point="140,80" Size="20,20" SweepDirection="Clockwise"/>
                        <LineSegment Point="70,80"/>
                        <LineSegment Point="70,130"/>
                        <LineSegment Point="40,80"/>
                        <LineSegment Point="20,80"/>
                        <ArcSegment Point="0,60" Size="20,20" SweepDirection="Clockwise"/>
                        <LineSegment Point="0,20"/>
                        <ArcSegment Point="20,0" Size="20,20" SweepDirection="Clockwise"/>
                    </PathFigure>
                </PathGeometry>
            </Path.Data>

            <Path.RenderTransform>
                <ScaleTransform ScaleX="1.3" ScaleY="1.3"/>
            </Path.RenderTransform>
        </Path>

        <StackPanel Margin="5">
            <Button HorizontalAlignment="Right"
                    Margin="0,5,10,0"
                    Click="cmdClose_Click">x</Button>

            <TextBlock TextWrapping="Wrap"
                       FontSize="15"
                       HorizontalAlignment="Center"
                       PointerPressed="window_PointerPressed">
                This is a balloon-shaped window.
            </TextBlock>
        </StackPanel>

    </Grid>
</Window>

using Avalonia;
using Avalonia.Controls;
using Avalonia.Input;
using Avalonia.Interactivity;
using Avalonia.Markup.Xaml;
using Avalonia.Media;

namespace AvaloniaUI;

public partial class TransparentWithShapes : Window
{
    public TransparentWithShapes()
    {
        InitializeComponent();
        ApplyClip();
    }
    private void ApplyClip()
    {
        if (shapePath.Data is not Geometry geometry)
            return;

        Geometry clip;

        clip = Geometry.Parse(geometry.ToString() ?? "");

        if (shapePath.RenderTransform != null)
        {
            clip.Transform = new MatrixTransform(
                shapePath.RenderTransform.Value
            );
        }

        root.Clip = clip;
    }
    private void cmdClose_Click(object? sender, RoutedEventArgs e)
    {
        Close();
    }

    private void window_PointerPressed(object? sender, PointerPressedEventArgs e)
    {
        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
            BeginMoveDrag(e);
    }
}