I think the thing you see as an “M+”, I see it as M 
. That toggle is a new feature that replaced the old preview pane. The preview pane used to be a panel on the right which you could toggle on and off. The place you typed your post would appear on the left, and could be expanded to full screen with no preview pane. The editing pane on the left showed the raw text and all the markdown you might use. The right was how the post would appear after you finished it. They’ve changed things so there’s a toggle and you can’t see both views at once anymore.
ETA: The preview pane used to show you exactly how your post will appear; this supposed WYSIWYG toggle doesn’t show everything, for example when I tried to put that down arrow in there, it wasn’t showing correctly while I edited this. But now while I’m revising my post I do see the emoji there. Weird.
And it still is, it’s still there unless you hid it. Just like before when you could hide it. I use the preview pane every time. I’m using it with this post.
So what do we click now to get it back, the simultaneous view of what you’re typing in markdown, and the preview to the right of that? I’ve lost those controls that were there before.
As previously stated. It’s the same little double arrow we had before there was a choice between the WSYIWYG and markdown modes (basically, there was only a markdown mode before). If you are in markdown mode and don’t see it, click the arrows in the bottom-right corner to unhide it.
Going into markdown mode and hiding the preview pane is the best workaround when you’re trying to post a link to a video you want to embed in a post, so it’s good that we still have the ability to hide the pane, but if you forget to put it back you might forget it’s available.
Ahh.. thank you, that arrow to unhide the preview pane disappears when not in the markdown mode, and I was just missing it. Okay, so, my bad, I was confused.
Not your fault, the new UI is really different.
Because (the theory goes), you don’t need a preview pane when you’re using a “What You See Is What You Get” editor. Your editing panel is your preview.
Hence, no preview, and no button to summon the preview.
People who get confused need to just select the 
toggle and get back exactly the same behaviors they were used to. For a change, the Discourse devs (eventually) made all their new editing juju completely optional.
One more comment on the “similar OP’s” feature. …
The warning when you start a new OP that it is similar to existing OPs uses a completely different mechanism than does the “related topics” feature at the bottom of a page.
The selections at the bottom of the page are often pretty good. The warning while you’re composing your title is utterly brain damaged.