You can start a draft post (using the highlight -> Quote reply method) and then navigate to the other thread. When you hit Reply there will be a pop-up asking if you want to reply in the original thread, or at your new location.
You can start a draft post (using the highlight -> Quote reply method) and then navigate to the other thread. When you hit Reply there will be a pop-up asking if you want to reply in the original thread, or at your new location.