As on the sent gif I see that the mouse is not positioned on the DropDownList, but below the component, I would like to ask you to send us details on the performed steps. For example, after you open the page with the rendered DropDownList do you click on the DropDownList to focus it?
Could you please provide more details regarding how the issue can be replicated? Also please confirm if the issue occurs in our demos. I tried to replicate the issue, but to no avail. As you can see in the screencast linked below, the select event is not fired when clicking the middle mouse button.