vim-patch:9.1.0260: Problems with "zb" and scrolling to new topline with 'smoothscroll'

Problem: "zb" does not reveal filler lines at the start of a buffer.
          Scrolled cursor position with 'smoothscroll' is unpredictable,
          and may reset skipcol later if it is not visible (after v9.1.258)
Solution: Replace confusing for loop that reaches final control value too
          early with while loop. Set "w_curswant" accordingly so cursor
          will be placed in visible part of topline.
          (Luuk van Baal)

https://github.com/vim/vim/commit/bd28cae1f1c21c0e3743e3427c98bbd848fad237

4 files changed, 43 insertions, 22 deletions

diff --git a/src/nvim/move.c b/src/nvim/move.c
index 63505c72ab..a1c0a02837 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -1231,6 +1231,10 @@ bool scrolldown(win_T *wp, linenr_T line_count, int byfold)
         wp->w_topline--;
         wp->w_skipcol = 0;
         wp->w_topfill = 0;
+        // Adjusting the cursor later should not adjust skipcol.
+        if (do_sms) {
+          curwin->w_curswant = MAXCOL;
+        }
         // A sequence of folded lines only counts for one logical line
         linenr_T first;
         if (hasFolding(wp, wp->w_topline, &first, NULL)) {
@@ -1385,9 +1389,10 @@ bool scrollup(win_T *wp, linenr_T line_count, bool byfold)
           wp->w_topline = lnum;
           wp->w_topfill = win_get_fill(wp, lnum);
           wp->w_skipcol = 0;
-          // Adjusting the cursor later should not adjust skipcol:
-          // bring it to the first screenline on this new topline.
-          wp->w_curswant %= width1;
+          // Adjusting the cursor later should not adjust skipcol.
+          if (do_sms) {
+            curwin->w_curswant = 0;
+          }
           if (todo > 1 && do_sms) {
             size = linetabsize(wp, wp->w_topline);
           }
@@ -1846,15 +1851,11 @@ void scroll_cursor_bot(win_T *wp, int min_scroll, bool set_topbot)
   bool do_sms = wp->w_p_wrap && wp->w_p_sms;
 
   if (set_topbot) {
-    bool set_skipcol = false;
-
     int used = 0;
     wp->w_botline = cln + 1;
+    loff.lnum = cln + 1;
     loff.fill = 0;
-    for (wp->w_topline = wp->w_botline;
-         wp->w_topline > 1;
-         wp->w_topline = loff.lnum) {
-      loff.lnum = wp->w_topline;
+    while (true) {
       topline_back_winheight(wp, &loff, false);
       if (loff.height == MAXCOL) {
         break;
@@ -1872,25 +1873,23 @@ void scroll_cursor_bot(win_T *wp, int min_scroll, bool set_topbot)
             wp->w_topline = loff.lnum;
             wp->w_skipcol = skipcol_from_plines(wp, plines_offset);
             wp->w_cursor.col = wp->w_skipcol + overlap;
-            set_skipcol = true;
           }
         }
         break;
       }
-      used += loff.height;
       wp->w_topfill = loff.fill;
+      wp->w_topline = loff.lnum;
+      used += loff.height;
     }
-    if (wp->w_topline > wp->w_buffer->b_ml.ml_line_count) {
-      wp->w_topline = wp->w_buffer->b_ml.ml_line_count;
-    }
+
     set_empty_rows(wp, used);
     wp->w_valid |= VALID_BOTLINE|VALID_BOTLINE_AP;
     if (wp->w_topline != old_topline
         || wp->w_topfill != old_topfill
-        || set_skipcol
+        || wp->w_skipcol != old_skipcol
         || wp->w_skipcol != 0) {
       wp->w_valid &= ~(VALID_WROW|VALID_CROW);
-      if (set_skipcol) {
+      if (wp->w_skipcol != old_skipcol) {
         redraw_later(wp, UPD_NOT_VALID);
       } else {
         reset_skipcol(wp);
@@ -2305,7 +2304,8 @@ static int get_scroll_overlap(Direction dir)
   int min_height = curwin->w_height_inner - 2;
 
   validate_botline(curwin);
-  if (dir == FORWARD && curwin->w_botline > curbuf->b_ml.ml_line_count) {
+  if ((dir == BACKWARD && curwin->w_topline == 1)
+      || (dir == FORWARD && curwin->w_botline > curbuf->b_ml.ml_line_count)) {
     return min_height + 2;  // no overlap, still handle 'smoothscroll'
   }
 
@@ -2435,7 +2435,6 @@ int pagescroll(Direction dir, int count, bool half)
     } else {
       cursor_up_inner(curwin, count);
     }
-    curwin->w_curswant = prev_curswant;
 
     if (get_scrolloff_value(curwin)) {
       cursor_correct(curwin);
@@ -2453,12 +2452,13 @@ int pagescroll(Direction dir, int count, bool half)
     nochange = scroll_with_sms(dir, &count);
   }
 
+  curwin->w_curswant = prev_curswant;
   // Error if both the viewport and cursor did not change.
   if (nochange) {
     beep_flush();
   } else if (!curwin->w_p_sms) {
     beginline(BL_SOL | BL_FIX);
-  } else if (p_sol) {
+  } else if (p_sol || curwin->w_skipcol) {
     nv_g_home_m_cmd(&ca);
   }
 
diff --git a/test/functional/legacy/normal_spec.lua b/test/functional/legacy/normal_spec.lua
index 113f7c668a..c57e51c18e 100644
--- a/test/functional/legacy/normal_spec.lua
+++ b/test/functional/legacy/normal_spec.lua
@@ -94,10 +94,9 @@ describe('normal', function()
     feed('ggG<C-D>')
     screen:expect({
       grid = [[
-        foobar one two three                    |*8
+        foobar one two three                    |*16
         ^foobar one two three                    |
         {2:---}                                     |
-        {1:~                                       }|*8
                                                 |
       ]],
     })
diff --git a/test/old/testdir/test_normal.vim b/test/old/testdir/test_normal.vim
index 013d8959ca..10fbf4125a 100644
--- a/test/old/testdir/test_normal.vim
+++ b/test/old/testdir/test_normal.vim
@@ -4220,4 +4220,19 @@ func Test_single_line_scroll()
   call prop_type_delete(vt)
 endfunc
 
+" Test for zb in buffer with a single line and filler lines
+func Test_single_line_filler_zb()
+  call setline(1, ['', 'foobar one two three'])
+  diffthis
+  new
+  call setline(1, ['foobar one two three'])
+  diffthis
+
+  " zb scrolls to reveal filler lines at the start of the buffer.
+  exe "normal \<C-E>zb"
+  call assert_equal(1, winsaveview().topfill)
+
+  bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable
diff --git a/test/old/testdir/test_scroll_opt.vim b/test/old/testdir/test_scroll_opt.vim
index 4e551cdaad..d381456032 100644
--- a/test/old/testdir/test_scroll_opt.vim
+++ b/test/old/testdir/test_scroll_opt.vim
@@ -1021,6 +1021,8 @@ func Test_smoothscroll_page()
   call assert_equal(0, winsaveview().skipcol)
 
   " Half-page scrolling does not go beyond end of buffer and moves the cursor.
+  " Even with 'nostartofline', the correct amount of lines is scrolled.
+  setl nostartofline
   exe "norm! 0\<C-D>"
   call assert_equal(200, winsaveview().skipcol)
   call assert_equal(204, col('.'))
@@ -1044,7 +1046,7 @@ func Test_smoothscroll_page()
   call assert_equal(204, col('.'))
   exe "norm! \<C-U>"
   call assert_equal(0, winsaveview().skipcol)
-  call assert_equal(1, col('.'))
+  call assert_equal(40, col('.'))
 
   bwipe!
 endfunc
@@ -1062,6 +1064,11 @@ func Test_smoothscroll_next_topline()
   redraw
   call assert_equal(0, winsaveview().skipcol)
 
+  " Also when scrolling back.
+  exe "norm! G\<C-Y>"
+  redraw
+  call assert_equal(880, winsaveview().skipcol)
+
   " Cursor in correct place when not in the first screenline of a buffer line.
   exe "norm! gg4gj20\<C-D>\<C-D>"
   redraw