High CPU usage with navigator on

In MuseScore 2.0.3, we reworked the navigator and shows the scrollbar when needed. Unfortunately it causes an issue because the pages need to be scale differently with or without the scrollbar. In the worse scenario, the pages don't fit, and so we need a scrollbar, but then they fit, and we don't need the scrollbar etc... loop, and CPU usage goes up.

See also #21247: Regression—scroll bar for Navigator always appears