patch 9.1.1388: Scrolling one line too far with 'nosmoothscroll' page scrolling

Problem:  One-off error in "count" to make "w_skipcol" zero with
          'nosmoothscroll' page scrolling when last virtual line
          in a buffer line is exactly the entire window width.
          (Hirohito Higashi)
Solution: Properly compute the smallest integer value necessary
          to make "w_skipcol" zero (Luuk van Baal)

fixes: #17317
closes: #17318

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 7f4f909..069651f 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -4354,4 +4354,21 @@
   bwipe!
 endfunc
 
+" Test Ctrl-B with 'nosmoothscroll' not stuck with line exactly window width.
+func Test_scroll_longline_winwidth()
+  10new
+  call setline(1, ['']->repeat(20) + ['A'->repeat(20 * winwidth(0))] + ['']->repeat(20))
+  exe "normal! G3\<C-B>"
+  call assert_equal(22, line('w0'))
+  exe "normal! \<C-B>"
+  call assert_equal(21, line('w0'))
+  exe "normal! \<C-B>"
+  call assert_equal(11, line('w0'))
+  exe "normal! \<C-B>"
+  call assert_equal(3, line('w0'))
+  exe "normal! \<C-B>"
+  call assert_equal(1, line('w0'))
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable