patch 9.1.0215: Half-page scrolling does not support smooth-scrolling

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)

closes: #14316

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index dd2f2be..3a7248f 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -1280,9 +1280,13 @@
   exe "normal \<C-D>"
   call assert_equal(46, line('.'))
   exe "normal \<C-U>"
-  call assert_equal(36, line('.'))
+  call assert_equal(36, line('w0'))
+  call assert_equal(46, line('.'))
   exe "normal \<C-U>"
-  call assert_equal(10, line('.'))
+  call assert_equal(1,  line('w0'))
+  call assert_equal(40, line('.'))
+  exe "normal \<C-U>"
+  call assert_equal(30, line('.'))
   exe "normal \<C-U>"
   call assert_equal(1, line('.'))
   set scroll&
@@ -1303,9 +1307,8 @@
   call assert_equal(50, line('.'))
   call assert_equal(100, line('w$'))
   normal z.
-  let lnum = winline()
   exe "normal \<C-D>"
-  call assert_equal(lnum, winline())
+  call assert_equal(1, winline())
   call assert_equal(50, line('.'))
   normal zt
   exe "normal \<C-D>"
@@ -3066,7 +3069,8 @@
   call assert_equal(2, &scroll)
   set scroll=5
   exe "norm! \<c-u>"
-  call assert_equal('3', getline('.'))
+  call assert_equal('3', getline('w0'))
+  call assert_equal('8', getline('.'))
   1
   set scrolloff=5
   exe "norm! \<c-d>"
@@ -3813,11 +3817,11 @@
   call assert_equal(11, line('.'))
   call assert_equal(1, winline())
   exe "normal \<C-B>"
-  call assert_equal(11, line('.'))
-  call assert_equal(9, winline())
+  call assert_equal(10, line('.'))
+  call assert_equal(10, winline())
   exe "normal \<C-B>\<C-B>"
   call assert_equal(5, line('.'))
-  call assert_equal(1, winline())
+  call assert_equal(5, winline())
 
   bwipe!
 endfunc
@@ -4186,12 +4190,16 @@
   norm! {
   call assert_equal([0, 1, 1, 0], getpos('.'))
 
-  " Ctrl-B scrolls up with hidden "above" virtual text.
+  " Ctrl-B/Ctrl-U scroll up with hidden "above" virtual text.
   set smoothscroll
   exe "normal \<C-E>"
   call assert_notequal(0, winsaveview().skipcol)
   exe "normal \<C-B>"
   call assert_equal(0, winsaveview().skipcol)
+  exe "normal \<C-E>"
+  call assert_notequal(0, winsaveview().skipcol)
+  exe "normal \<C-U>"
+  call assert_equal(0, winsaveview().skipcol)
 
   set smoothscroll&
   bw!