pageUpAndModifySelection: doesn't always center selection in viewport

[davidbalbert]

To reproduce

1. Drag the scroller and scroll down to the bottom of the document
2. Click to put the insertion point into the viewport
3. Press Fn-Shift-Up multiple times.

Expected result

The head of the selection is always in the viewport.

Actual result

Sometimes the selection is not visible in the viewport.

This bug was present before the big scroll-correction refactoring that introduced ScrollManager (#128), but it seems worse now than it was before.