Click the link button a few times. Notice the page isn't refreshed.
It would be great to have a toggle between the current above/below placement of the preview/editor, and a left/right placement for those with wide screens. A splitter in the middle would also be great.
Most of the time I'd use the code editor, but there's one exception: cut'n pasting text with hyperlinked substrings. Way easier to do in a WYSIWYG editor than pasting the raw text, then formatting each hyperlink (by copying the hyperlink from the source)