<DataTrigger Binding="{Binding IsMouseOver, ElementName=minimapButton}" Value="True">
<Setter Property="IsOpen" Value="True" />
</DataTrigger>
The binding above is using IsMouseOver which refer to the element name minimapButton.
What should be the code for mouse click? IsMouseClicked? IsMouseLeftDown?
Register a new event for the button click.
And the code behind