Window and Page View Buttons
In Window View, Composer arranges the pages of the score as though the page size were exactly the size of the window. If you change the window size, Composer will automatically and quickly repaginate the score for the new "page size", which is, in this case, the window size.
In Page View, Composer displays an image of the page as it would be printed, which is usually larger than what can be seen all at once in the window. Therefore, in Page View, Composer usually displays scrollbars that enable you to scroll horizontally and/or vertically in the page. In Page View you can view the entire printed page by using the Zoom Out command.
You can toggle between Window View and Page View using these two buttons in the toolbar:
For more information, see the topic on Window View and Page View.