At the top of the small screen you described in the corner, there is an "X," the symbol microsoft uses for "close." next to that, there is a "+" and next to that there is a blue "^"
The "^" is the symbol to slide up. Click the blue "^" and it will slide the larger window up and get rid of whatever this preview thing is.
What is Markdown?
Markdown lets you add more formatting to your post. Simply type in your post and it will display as written.
If you wish to add bold or italic characters, add a hyperlink to another website, a heading or a horizontal line, simply use the relevent icons above the text input field.
A preview of your post will appear in the grey box below. If you make a change and you're not happy, simply press the back arrow icon to undo.