Timeline of events: Entered merchant dialogue, purchased Legendary box, went to go to inventory and accidentally clicked into account overview on this tab, hit back, selected "nevermind" in merchant dialogue, failed to be able to interact with any buttons in the explore window. Tried refreshing the page and re-entering through the Seasonal Festival tab and neither of these solved the issue.

Relogging showed that I was trying to talk to the merchant again, but with the buttons greyed out prior, I could not get back into the dialogue to try and leave.

It may have had to do with the fact that I accidentally left the window while still in the merchant dialogue. Still, it was weird. (I'm glad someone suggested relogging, I'm too dull to think of that myself.)