when zooming: don't show scrollbars when not needed

[ralf3u]

If I zoom in the content-area of the Preview on a touch-device, then there are scrollbars even if scrollbars are not necessary. In the browser Falcon that uses the same engine like Preview, when zooming then there are no scrollbars when no scrollbars are necessary.

[xiota]

I probably won't change this unless I see it can be done with a built-in option. I don't want to have to track down new bugs from introducing complicated logic.