When you click "Menu: View/Article only" (or use the corresponding toolbar button, or the "F12" key on your keyboard) the tree will be hidden and the article-area will fill the entire TreePad window.
You can always restore the default view (Tree and article) using "Menu: View/Article and tree" (or use the corresponding toolbar button, or press "F10" on your keyboard).
You can to use the Article-only View when e.g. you want to focus on working with the article for a while (use TreePad as a text editor) and need the entire screen to do this.
Please note that in the "Article only" view you still can use the tree navigate functions ("Menu: Navigate/Go up", "Menu: Navigate/Go down", "Menu: Navigate/Go left", "Menu: Navigate/Go right" and their corresponding toolbar buttons).