David Hurka authored Apr 04, 2021 and Nate Graham committed Jul 22, 2021

PageView::slotScrollUp/Down() queried the size of the scrollbar instead of the viewport,
which has no obvious reason and doesn’t work when scrollbars are not there.

Probably the scrollbar size was thought to be more robust
in times when PageView had overshoot enabled.

Also fixes code documentation for these methods,
and removes another spaghetti feature where Continuous mode was queried
in an `if` branch before the one where it is relevant.

BUG: 421822