MainWindow.xaml

<Window x:Class="ContextMenu.OpenBehavior.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        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"
        xmlns:b="http://schemas.microsoft.com/xaml/behaviors" 
        xmlns:local="clr-namespace:ContextMenu.OpenBehavior"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Button VerticalAlignment="Center"
                HorizontalAlignment="Center"
                Content="Hover Over Me"
                x:Name="Button">
            <b:Interaction.Behaviors>
                <local:OpenContextMenuBehavior OpenContextMenu="{Binding IsMouseOver, ElementName=Button}" />
            </b:Interaction.Behaviors>
            <Button.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="Item 1" />
                    <MenuItem Header="Item 2" />
                    <MenuItem Header="Item 3" />
                    <MenuItem Header="Item 4" />
                    <MenuItem Header="Item 5" />
                </ContextMenu>
            </Button.ContextMenu>
        </Button>
    </Grid>
</Window>