Sometimes you may want more space to edit in the main pane, or you accidentally hide the page tree. Below is how to show or hide the page tree.
The image below shows how to hide the page tree. Simply click in the area circled in red: when you get your mouse into that area you will see a tab appear as in the picture. Just click to hide the page tree.
To show the page tree after it has been hidden, you can do the same thing. Mouse over the divide circled in red and then click.