As Nathan mentioned in https://reviews.mahara.org/3326, when you set a forum's indentation to "no indent", we still show a "reply" button next to each post. This is confusing, because there's no visible difference depending on which one you click "reply" to.
Basically, changing the indentation to "no indent" changes the behavior of the forum from threaded to single-thread. So we should reflect that. There should be a "post" or "reply" button at the top of the page and at the bottom, and your post should always be set as the reply to the current last post in the topic.
Perhaps we ought to change the text description of the "indent" setting as well, to reflect the fact that now it's about behavior as well as display?
Patch for "master" branch: https:/ /reviews. mahara. org/3516