This is an attached behavior that will show the `ContextMenu` for a Button control when left-clicked.

LeftClickContextMenuBehavior.cs

// LeftClickContextMenuBehavior.cs,  4/30/2015 5:49:44 PM

using System;
using System.Windows;
using System.Windows.Controls;
namespace Common.AttachedBehaviors
{
    public static class LeftClickContextMenuBehavior
    {
        private static readonly DependencyProperty LeftClickContextMenuProperty = DependencyProperty.RegisterAttached(
            "Enabled", typeof(bool), typeof(LeftClickContextMenuBehavior), new PropertyMetadata(new PropertyChangedCallback(OnPropertyChanged))
            );

        public static bool GetEnabled(DependencyObject obj)
        {
            return (bool)obj.GetValue(LeftClickContextMenuProperty);
        }

        public static void SetEnabled(DependencyObject obj, bool value)
        {
            obj.SetValue(LeftClickContextMenuProperty, value);
        }

        private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if(d is Button)
            {
                var button = (Button)d;
                button.Click -= OnMouseClick;
                button.Click += OnMouseClick;
            }
        }

        private static void OnMouseClick(object sender, RoutedEventArgs e)
        {
            var obj = sender as DependencyObject;
            bool enabled = (bool)obj.GetValue(LeftClickContextMenuProperty);
            if (!enabled)
                return;
            
            Button b = sender as Button;
            if (HasContextMenu(b))
            {
                b.ContextMenu.PlacementTarget = b;
                b.ContextMenu.IsOpen = true;
            }
        }

        private static bool HasContextMenu(Button b)
        {
            return b != null && b.ContextMenu != null;
        }
    }
}

Usage:

<!-- controls is the namespace containing the behavior -->
<Button Style="{StaticResource ButtonStyle}" controls:LeftClickContextMenuBehavior.Enabled="True"/>