In AvaloniaUI, how to subscribe to mouse events of the titlebar

To recreate what I have done, create a new AvaloniaUI project. In the MainWindow.cs, get the TitleBar object by calling this.FindDescendantOfType<TitleBar>();in protected override void OnApplyTemplate(TemplateAppliedEventArgs e). Then add any event to its PointerPressed event.
However, the break point of the event handler method is never reached.

You cannot do it that way. If you inspect that TitleBar you will se its actually a generic object with no size or function, just existing in the tree (not sure why they don't just give you a null, probably some internal nullable issue).
Anyway, the reason you cannot is because chrome is OS based and controlled by OS and implementation is OS specific. Alternative is to build your own title bar and hide the system one, providing necessary features yourself (check this for ideas => Custom TitleBar Template).

If you do not want to do that and you are only doing Windows app, you can spy on your own message queue (safe since there is no DLL injection and GUI thread will not move across unmanaged threads) to detect "nonclient", e.g. titlebar click message (WM_NCLBUTTONDOWN) and react. For example:

MainWindow.axaml

<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"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="Test01.MainWindow"
        Title="Test01" >
	<Grid>
        <Label Name="MainLabel"/>
    </Grid>
</Window>

MainWindow.axaml.cs

using Avalonia.Controls;
using System;
using System.Runtime.InteropServices;

namespace Test01;

public partial class MainWindow : Window
{
    private const int WM_NCLBUTTONDOWN = 0x00a1;
    private const int WH_GETMESSAGE = 3;

    private IntPtr theHook;

    public MainWindow()
    {
        InitializeComponent();
    }

    protected override void OnInitialized()
    {
        base.OnInitialized();
        theHook = MyInterop.SetWindowsHookEx(WH_GETMESSAGE, MsgHook, IntPtr.Zero, MyInterop.GetCurrentThreadId());
        // var err = Marshal.GetLastWin32Error();
    }

    private int MsgHook(int nCode, IntPtr wParam, IntPtr lParam)
    {
        // mandatory shortcut
        if (nCode < 0) return MyInterop.CallNextHookEx(theHook, nCode, wParam, lParam);
        // is it a NC click?
        var msg = (MyInterop.MsgStruct?)Marshal.PtrToStructure(lParam, typeof(MyInterop.MsgStruct));
        if (msg?.message == WM_NCLBUTTONDOWN)
        {
            // your action
            MainLabel.Content = "I am pressed!!!";
        }

        return MyInterop.CallNextHookEx(theHook, nCode, wParam, lParam);
    }

}

MyInterop.cs

namespace Test01;

internal static class MyInterop
{
    [StructLayout(LayoutKind.Sequential)]
    public class MsgStruct
    {
        public IntPtr hwnd;
        public uint message;
        // we dont care about the rest of MSG struct
    }


    internal delegate int HookProc(int code, IntPtr wParam, IntPtr lParam);

    [System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "SetWindowsHookEx", SetLastError = true)]
    internal static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint dwThreadId);

    [System.Runtime.InteropServices.DllImport("user32.dll")]
    internal static extern int CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

    [DllImport("kernel32.dll")]
    internal static extern uint GetCurrentThreadId();
}

This is just a working example, you can repackage it into some observable pattern or event launcher.

Also, this is just for Windows, wont work on Linux and MacOS. For X11, there is also a message queue so I think alternative may be possible. Not at all sure about MacOS.

NOTE: I had to move all extern definitions to a separate class because of the bug in Avalonia (Xaml Compiler error when code-behind class contains a DllImport method #10046). Once they fix that MyInterop can be merged into main .cs