I think you are toggling between rich text editor (first screenshot) and Markdown viewer (second screenshot). You can try to press ctrl + L or the "toggle layout" button at top right while you are at the Markdown viewer.
In fact the Mardown Editor and his viewer and the Rich Text Editor and his viewer, and the fact to be able to split, and so on, is something a bit disturbing at the begining for many users.
Here are the commands in order to "play" with the thing: