Using any Invoice module, you can toggle the display of the object bar window on the left side of the screen. When you are using the Invoice Manager module, this means you can view invoice definitions on the entire screen rather than just part of the screen.
To do this, deselect in the menu. The items on the left side of the screen are hidden. Select again to view them.
You can also change the proportions of the display by using your mouse to drag the bar dividing the two halves of the display. How?