Script Debugger lets you choose between wrapping long lines automatically and letting long lines extend to the right. To toggle line wrapping:

• Click the wrap icon (at the top of the vertical scroll bar), or choose View > Wrap Lines. If the menu item is checked, line wrapping is turned on (). If not, line wrapping is turned off (), and long lines will extend to the right and a horizontal scroll bar will appear if necessary.

Each setting has its merits. AppleScript is a line-based language, so when you’re viewing AppleScript code it can be nice to let long lines extend to the right, so that “a line is a line is a line”. On the other hand, scrolling horizontally to see long lines is troublesome, and wrapping solves the problem nicely.