Git hangs on mergetool prompt

When a merge conflict is detected, git naturally runs the mergetool. I haven't specified a certain mergetool in my Git config, so it tries all three and settles on vimdiff:

It then hangs, waiting for the "user" (Qt Creator) to hit return. As far as I can tell, there's no way for me to initiate this. In the Qt Creator docs it unhelpfully says to run the merge tool using Tools > Git > Git Tools > Merge Tool, which doesn't seem to exist.

This leaves my repo in limbo since vimdiff is running and a merge is pending but nothing happens. Task Manager clearly shows the applications are waiting for user input but I don't see a way to finish it.

Is this a Qt Creator bug or user error?