sean Van Flarum doesn't have that option natively, but I've located a plugin that can do it. The plugin is now active, you use it via the "eye" icon that appears next to the Post button. Hope this helps 🙂
Van Thanks sean. I also noticed a few seconds ago while applying to another one of your responses that clicking the minimize (-) icon will also show a preview of your post while in edit mode.
sean Van that works for replies, but not for new posts. The preview plugin that I added will let you preview new posts directly in the composer window.