This will only happen if your new instance is opened to the same topic as the one in which you’re composing a reply. Instead try something like Ctrl+Click (Cmd+Click) on the site logo, you’ll get a new tab on the topic list, and the composer window will not be open in the new tab.
I think that should address the main issue here.
The only other cases I can find where this might break down are:
- when you attempt to reply to 2 separate posts within the same topic.
- when you attempt to do “Reply as new Topic” in two different topics.
For case #1, the recommended practice is to use the quote-reply feature and make one post.
There’s no current workaround for case #2:
There are a number of topics discussing this so its clear there’s a desire among some segment of users to maintain multiple new topic drafts. Personally, I find it rarely needed though there have been a few occasions where I wish that feature existed.