Is there a possibility in AvaloniaUI, to globally capture mouse press button up/down? To be notified about this events outside of any controls (or possibly outside of any particular view model)?
You can listen for these Events in the Code-Behind of your MainWindow and also handle Events that have already been handled by other Controls with handledEventsToo: true like this:
public class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new MainWindowViewModel();
        this.AddHandler(PointerPressedEvent, MouseDownHandler, handledEventsToo: true);
        this.AddHandler(PointerReleasedEvent, MouseUpHandler, handledEventsToo: true);
        #if DEBUG
        this.AttachDevTools();
        #endif
    }
    private void MouseUpHandler(object sender, PointerReleasedEventArgs e)
    {
        System.Diagnostics.Debug.WriteLine("Mouse released.");
    }
    private void MouseDownHandler(object sender, PointerPressedEventArgs e)
    {
        System.Diagnostics.Debug.WriteLine("Mouse pressed.");
    }
    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
    }
}
Note that this (probably) won't work globally if you have multiple windows.
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