IsDropDown on IsMouseOver of ComboBox is killed by ONE mouseclick on WPF ComboBox WHEN it opens

Question

This is the Style set to my ComboBox. When I hover the ComboBox, the popup opens/slides and then right at the moment where it opens I click on it which is closing the combobox because I have choosed an item. The next time I hover the combobox is does not open anymore...

Why?

<Style x:Key="GroupViewModelStyle">
            <Style.Triggers>
                <Trigger Property="ComboBox.IsMouseOver" Value="True">
                    <Setter Property="ComboBox.IsDropDownOpen" Value="True" />
                </Trigger>
                <Trigger Property="ComboBox.IsMouseOver" Value="False">
                    <Setter Property="ComboBox.IsDropDownOpen" Value="False" />
                </Trigger>
            </Style.Triggers>
        </Style>