If Main menu: View⁄Wrap text to page is selected, the text is wrapped to the page (as defined in Main menu: View⁄Options, category: Page setup). This means that the width of the line is decided by you, not by the size of the window. Use this to make the article conform to standard letter sizes when the article is to be printed. However, the right margin of the text may extend beyond the Article pane unless you widen the pane.
If Main menu: View⁄Wrap text to screen is selected, the text is automatically adjusted ("wrapped") when it reaches the right edge of the Article pane so that it is always visible, no matter how small the window.