When the Search tool in the header is clicked, it blocks all dropdowns (All Communities, Community Basics, and profile). You have to click the Search tool again to get access to them. Clicking on the links (Community, ArcGIS Ideas, Mail, and Notifications) still work.
Thanks for the feedback Ken. This is a very good spot. We applied the same logic as Esri.com search takeover but on their site I can see that when you click a global link that has a dropdown/megamenu it automatically minimizes search again and shows you the dropdown/megamenu. We would look to emulate that.
We'll look at getting this addressed in our Phase 2 enhancements on our megamenu and global nav behaviors.