Stop Dropdown menu opening when opening site

On one of my sites, a dropdown menu opens when the site is being opened -- eventually it closes, but what is causing this, and so how do I stop it happening, please?

Anyone familiar with FOUC -- could this be an example of it, or is it just a bug?

It is likely that you have set the Dropdown to Expanded in the app itself. You can see the setting to switch it off in the Options panel:

Options Panel