How about showing the preivew in-line with the discussion?
When you click Reply or Save Edit, all that happens is the border (or whatever styling you use to show that the post is being modified) disappears. Very smooth.
This could even work on mobile.
Maybe keep the “show preview” button in case you get lost while scrolling around the topic and need to get back to your post.
Only downside is that you can’t reference the rest of the topic and watch the preview at the same time. You have to scroll back to the preview to see the effects of your edits (should this happen automatically when you type?). But I feel this is a less painful context switch than jumping between compose and preview.