Generally, WPF ContextMenu is opened right click on a control. Any user don't like right click. I try to open ContextMenu left clicking. DataContext is not binding using right click for some reason. This is simple solution for that problem.

MainWindow.xaml

<Window x:Class="ContextMenuBindingSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        
    </Window.Resources>
    <Grid>
        <Button MouseRightButtonUp="Button_MouseRightButtonUp" PreviewMouseLeftButtonDown="Button_MouseLeftButtonDown" Content="StaticContext" HorizontalAlignment="Left" Margin="29,29,0,0" VerticalAlignment="Top" Width="125" Height="68">
            <Button.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="{Binding StartName}" Command="{Binding StartCommand}"/>
                    <MenuItem Header="{Binding EndName}" Command="{Binding EndCommand}"/>
                </ContextMenu>
            </Button.ContextMenu>
        </Button>
        
    </Grid>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new MainWindowViewModel() { StartName = "start menu", EndName = "end menu" };
    }

    private void Button_MouseRightButtonUp(object sender, MouseButtonEventArgs e)
    {
        e.Handled = true;
    }

    private void Button_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        var button = sender as Button;
        if (button != null)
        {
            // Why is button.ContextMenu.DataContext null? ... when right click is O.K. but leftdown is N.G.
            button.ContextMenu.DataContext = button.DataContext;
            button.ContextMenu.IsOpen = true;
        }
    }
}

MainWindowViewModel.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;

namespace ContextMenuBindingSample
{
    class MainWindowViewModel : INotifyPropertyChanged
    {
        private string _StartName;
        public string StartName
        {
            get
            {
                return _StartName;
            }
            set
            {
                _StartName = value;
                NotifyPropertyChanged("StartName"); 
            }
        }

        public ICommand _StaftCommand;
        public ICommand StartCommand
        {
            get
            {
                if (_StaftCommand == null)
                {
                    _StaftCommand = new StartCommand();
                }
                return _StaftCommand;
            }
        }

        public ICommand _EndCommand;
        public ICommand EndCommand
        {
            get
            {
                if (_EndCommand == null)
                {
                    _EndCommand = new EndCommand();
                }
                return _EndCommand;
            }
        }


        private string _EndName;
        public string EndName
        {
            get
            {
                return _EndName;
            }
            set
            {
                _EndName = value;
                NotifyPropertyChanged("EndName");
            }
        }

            
        public event PropertyChangedEventHandler PropertyChanged;


        private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }



    }
    
    
    
    class StartCommand : ICommand
    {
        public bool CanExecute(object parameter)
        {
            return true;
        }

        public event EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            MessageBox.Show("StartCommand");
        }
    }

    class EndCommand : ICommand
    {
        public bool CanExecute(object parameter)
        {
            return true;
        }

        public event EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            MessageBox.Show("EndCommand");
        }
    }

}