Problem: Page-wise scrolling with Ctrl-D/Ctrl-U implements
it's own logic to change the topline and cursor.
More logic than necessary for scrolling with Ctrl-F/Ctrl-B
was removed in patch 9.1.0211.
Solution: Re-use the logic from Ctrl-E/Ctrl-Y/Ctrl-F/Ctrl-B while
staying backward compatible as much as possible.
Restore some of the logic that determined how many lines will
be scrolled (Luuk van Baal)
5a2e3ec9ac
2f638c0ac6
