You haven’t said which version of Omnis 7 you’re using.
I did find mention of your exact issue on the mailing list archive (http://www.omnis-dev.com:8090/searchform.do) though.
13 March 2016:
I dont think there are any way to get them to work with O18.104.22.168 since Windows 8, under windows 7 you could turn off Desktop composition to get them to work but this option is not available with newer versions of Windows.
On the good side I can also confirm that they work fine with O7.3.8
In reply to:
I have some problems to click in a dropdown list with my mouse.I can only use the arrows.
Is there a solution to this problem ?
A patch was available on the omnis.net ftp service at the time.