When I click on the inbox icon in the top bar, there's a region 'selected' which is larger than the square connected to the drop-down menu, extending so far to the right that it nearly hits the achievements icon. Clicking again to make the drop-down menu disappear leaves this region still selected:
The same happens when I select the achievements icon, only the selected region goes even further out to the right:
This looks like a bug left over from the Winter Bash, when there were three buttons up there with drop-down menus rather than just two; it looks as though the space used for those three has now stretched out to be covered by the remaining two.
Is this something the dev team are aware of?
I'm seeing this effect in Firefox, but not in Chrome.
EDIT 2: and as soon as I type that, I realise that I am still seeing it after all! It's just harder to get now - I need to switch between tabs or windows and back again in order to see the effect (which is why I didn't notice this until I'd edited this answer and then returned to my previous screen).