SimpleDraw/Views/MainWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:SimpleDraw.ViewModels"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:models="clr-namespace:SimpleDraw.Models"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="SimpleDraw.Views.MainWindow"
        xmlns:i="using:Avalonia.Xaml.Interactivity"
        xmlns:behaviors="using:Avalonia.Xaml.Interactions.Core"
        xmlns:componentModel="clr-namespace:CommunityToolkit.Mvvm.ComponentModel;assembly=CommunityToolkit.Mvvm"
        xmlns:system="clr-namespace:System;assembly=System.Runtime"
        x:DataType="vm:MainWindowViewModel"
        TransparencyLevelHint="Transparent"
        TransparencyBackgroundFallback="DimGray"
        Background="Transparent"
        Icon="/Assets/avalonia-logo.ico"
        Title="SimpleDraw"
        Width="{Binding WindowWidth}"
        Height="{Binding WindowHeight}"
        MinWidth="400"
        MinHeight="300"
        WindowStartupLocation="CenterScreen"
        >
    <Window.Resources>
        <system:String x:Key="BrushSizeFormat">{0:0.#####}</system:String>
    </Window.Resources>
    
    <Design.DataContext>
        <!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
        <vm:MainWindowViewModel/>
    </Design.DataContext>

    <Panel Background="Transparent">
        <SplitView Background="Transparent" IsPaneOpen="{Binding IsSplitOpen, Mode=TwoWay}" DisplayMode="Overlay" OpenPaneLength="250">
            <SplitView.Pane>
                <Border Background="Transparent" BorderBrush="Gray" BorderThickness="2">
                    <Grid Background="Transparent">
                        <Grid Background="Transparent">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="25"></RowDefinition>
                                <RowDefinition Height="50*"></RowDefinition>
                                <RowDefinition Height="50*"></RowDefinition>
                                <RowDefinition Height="50*"></RowDefinition>
                            </Grid.RowDefinitions>
                            <DockPanel Grid.Row="0" Background="Transparent">
                                <Menu DockPanel.Dock="Top" Background="Transparent" HorizontalAlignment="Left">
                                    <MenuItem Header="_File">
                                        <MenuItem Header="_Open..."/>
                                        <Separator/>
                                        <MenuItem Header="_Exit"/>
                                    </MenuItem>
                                    <MenuItem Header="_Edit">
                                        <MenuItem Header="Copy"/>
                                        <MenuItem Header="Paste"/>
                                    </MenuItem>
                                </Menu>
                            </DockPanel>

                            <StackPanel Grid.Row="1">
                                <TextBox Text="Select Color:" IsHitTestVisible="False" IsReadOnly="True" BorderThickness="0" Background="Transparent"/>
                                <ItemsControl ItemsSource="{Binding ColorPalette}">
                                    <ItemsControl.ItemsPanel>
                                        <ItemsPanelTemplate>
                                            <WrapPanel Orientation="Horizontal" />
                                        </ItemsPanelTemplate>
                                    </ItemsControl.ItemsPanel>
                                    <ItemsControl.ItemTemplate>
                                        <DataTemplate x:DataType="models:ColorPickerButton">
                                            <Button Margin="5,0,0,5"
                                                    BorderBrush="DimGray"
                                                    BorderThickness="2"
                                                    CornerRadius="12.5"
                                                    Background="{Binding Color}"
                                                    Width="20"
                                                    Height="20"
                                                    Command="{Binding SelectColorCommand}"
                                                    CommandParameter="{Binding}"
                                                    />
                                        </DataTemplate>
                                    </ItemsControl.ItemTemplate>
                                </ItemsControl>
                                <TextBox HorizontalAlignment="Left" TextAlignment="Left" Margin="5,5,5,0" IsHitTestVisible="False" IsReadOnly="True" BorderThickness="0" Background="Transparent">
                                    Brush Size:
                                </TextBox>
                                <TextBox Text="{Binding BrushThickness, Mode=TwoWay}" Margin="5,0" HorizontalAlignment="Stretch" TextAlignment="Left" BorderThickness="2" Background="Transparent"></TextBox>
                                <Slider Margin="5, 0" Value="{Binding BrushThickness}" Maximum="50" Minimum="0.5"></Slider>
                            </StackPanel>
                        </Grid>
                    </Grid>
                </Border>
            </SplitView.Pane>
                <Canvas Grid.Column="1" Background="White" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                    <i:Interaction.Behaviors>
                        <behaviors:EventTriggerBehavior EventName="PointerPressed">
                            <behaviors:InvokeCommandAction
                                Command="{Binding CanvasPointerPressedCommand}"
                                PassEventArgsToCommand="True" />
                        </behaviors:EventTriggerBehavior>
                        <behaviors:EventTriggerBehavior EventName="PointerMoved">
                            <behaviors:InvokeCommandAction
                                Command="{Binding CanvasPointerMovedCommand}"
                                PassEventArgsToCommand="True" />
                        </behaviors:EventTriggerBehavior>
                        <behaviors:EventTriggerBehavior EventName="PointerReleased">
                            <behaviors:InvokeCommandAction
                                Command="{Binding CanvasPointerReleasedCommand}"
                                PassEventArgsToCommand="True" />
                        </behaviors:EventTriggerBehavior>
                    </i:Interaction.Behaviors>

                    <ItemsControl ItemsSource="{Binding DrawingPoints}">
                        <ItemsControl.ItemsPanel>
                            <ItemsPanelTemplate>
                                <Panel HorizontalAlignment="Stretch" VerticalAlignment="Stretch" MinWidth="{Binding WindowWidth}" MinHeight="{Binding WindowHeight}"/>
                            </ItemsPanelTemplate>
                        </ItemsControl.ItemsPanel>
                        <ItemsControl.ItemTemplate>
                            <DataTemplate>
                                <Polyline Points="{Binding CurrentDrawingPoints}"
                                          Stroke="{Binding Brush}"
                                          StrokeThickness="{Binding BrushThickness}"
                                          >
                                </Polyline>
                            </DataTemplate>
                        </ItemsControl.ItemTemplate>
                    </ItemsControl>
                </Canvas>
        </SplitView>
        <Button Content="≡" Command="{Binding DisplaySplitEvent}" HorizontalAlignment="Left" VerticalAlignment="Top">
            <Button.RenderTransform>
                <TranslateTransform X="{Binding ButtonOffset}" />
            </Button.RenderTransform>
            <Button.Transitions>
            </Button.Transitions>
        </Button>
    </Panel>
</Window>