Make scrollbar in ARender previewer work within the same page

While the mouse scroll works properly, the vertical scrollbar in ARender previewer only works to move between pages.

• When previewing a file with only one page larger than the preview panel, the scrollbar is useless
• When previewing a file with more than one page, using the scrollbar only lets you move from one page to the beginning of other page, never to a specific place in the middle or end of a given page.