Automate mouse hover
To click an item of the drop-down menu, use the hover() method:

See the code below as an example:
<?xml version="1.0" encoding="UTF-8"?>
<config xmlns="http://web-harvest.sourceforge.net/schema/1.0/config" scriptlang="groovy">
<robotics-flow>
<robot name="r2d2" driver="internet explorer"
close-on-completion="true">
<script><![CDATA[
timeouts(40 * 1000)
open('https://rpa-tutorial.s3.amazonaws.com/trainings/dnd/samples/dhtmlxMenu/04_items/08_change_items_images.html')
$$('.top_level_text').get(0).hover()
$(byText('Open')).click()
$(byXpath('//*[@id="imgList"]/span[7]/img')).click()
sleep(2000)
$$('.top_level_text').get(1).hover()
$(byText('Select All')).click()
$('input').click()
sleep(2000)
$$('.top_level_text').each { item ->
item.hover()
sleep(2000)
}
]]></script>
</robot>
</robotics-flow>
<export include-original-data="false" />
</config>