I can replicate this on my test site. (107239a442)
- Create poll
- Test_User_One and Test_User_Two vote for the same poll option
- Merge Test_User_Two into Test_User_One
- Process gets stuck on ‘Merge Progress’ modal
Though saying that, after navigating away from the hanging modal, Test_User_Two appears to have vanished. I should have given them some posts to see if the merge was actually completed. Back in a minute. 
Very interestingly, Test_User_Two does not show up in my admin ‘active users’ list, but can still log in and post.
Update: It seems to have updated itself a little now, and Test_Two’s posts have been assigned to Test_One. I’m still logged in as Test_Two though, with a Test_Two Summary page, etc. I’ve even logged out, and logged back in again with Test_Two’s credentials.
Sorry, I was finding the quirks very interesting. They perhaps might not be useful.