vim-patch:9.0.1309: scrolling two lines with even line count and 'scrolloff' set

Problem:    Scrolling two lines with even line count and 'scrolloff' set.
Solution:   Adjust how the topline is computed. (closes vim/vim#10545)

https://github.com/vim/vim/commit/1d6539cf36a7b6d1afe76fb6316fe662f543bf60

Cherry-pick test_scroll_opt.vim changes from patch 8.2.1432.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

6 files changed, 74 insertions, 35 deletions

diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 129a8c6fb2..98832a98c9 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -1676,7 +1676,7 @@ void enter_buffer(buf_T *buf)
   maketitle();
   // when autocmds didn't change it
   if (curwin->w_topline == 1 && !curwin->w_topline_was_set) {
-    scroll_cursor_halfway(false);       // redisplay at correct position
+    scroll_cursor_halfway(false, false);  // redisplay at correct position
   }
 
   // Change directories when the 'acd' option is set.
diff --git a/src/nvim/move.c b/src/nvim/move.c
index a946633bf6..8fda73cbec 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -235,7 +235,7 @@ void update_topline(win_T *wp)
       // cursor in the middle of the window.  Otherwise put the cursor
       // near the top of the window.
       if (n >= halfheight) {
-        scroll_cursor_halfway(false);
+        scroll_cursor_halfway(false, false);
       } else {
         scroll_cursor_top(scrolljump_value(), false);
         check_botline = true;
@@ -314,7 +314,7 @@ void update_topline(win_T *wp)
         if (line_count <= wp->w_height_inner + 1) {
           scroll_cursor_bot(scrolljump_value(), false);
         } else {
-          scroll_cursor_halfway(false);
+          scroll_cursor_halfway(false, false);
         }
       }
     }
@@ -1464,7 +1464,7 @@ void scroll_cursor_top(int min_scroll, int always)
   // This makes sure we get the same position when using "k" and "j"
   // in a small window.
   if (used > curwin->w_height_inner) {
-    scroll_cursor_halfway(false);
+    scroll_cursor_halfway(false, false);
   } else {
     // If "always" is false, only adjust topline to a lower value, higher
     // value may happen with wrapping lines
@@ -1669,7 +1669,7 @@ void scroll_cursor_bot(int min_scroll, int set_topbot)
   // Scroll up if the cursor is off the bottom of the screen a bit.
   // Otherwise put it at 1/2 of the screen.
   if (line_count >= curwin->w_height_inner && line_count > min_scroll) {
-    scroll_cursor_halfway(false);
+    scroll_cursor_halfway(false, true);
   } else {
     scrollup(line_count, true);
   }
@@ -1690,7 +1690,7 @@ void scroll_cursor_bot(int min_scroll, int set_topbot)
 ///
 /// @param atend if true, also put the cursor halfway to the end of the file.
 ///
-void scroll_cursor_halfway(int atend)
+void scroll_cursor_halfway(bool atend, bool prefer_above)
 {
   int above = 0;
   int topfill = 0;
@@ -1705,38 +1705,59 @@ void scroll_cursor_halfway(int atend)
   loff.fill = 0;
   boff.fill = 0;
   linenr_T topline = loff.lnum;
+
   while (topline > 1) {
-    if (below <= above) {           // add a line below the cursor first
-      if (boff.lnum < curbuf->b_ml.ml_line_count) {
-        botline_forw(curwin, &boff);
-        used += boff.height;
+    // This may not be right in the middle if the lines'
+    // physical height > 1 (e.g. 'wrap' is on).
+
+    // Depending on "prefer_above" we add a line above or below first.
+    // Loop twice to avoid duplicating code.
+    bool done = false;
+    for (int round = 1; round <= 2; round++) {
+      if (prefer_above
+          ? (round == 2 && below < above)
+          : (round == 1 && below <= above)) {
+        // add a line below the cursor
+        if (boff.lnum < curbuf->b_ml.ml_line_count) {
+          botline_forw(curwin, &boff);
+          used += boff.height;
+          if (used > curwin->w_height_inner) {
+            done = true;
+            break;
+          }
+          below += boff.height;
+        } else {
+          below++;                    // count a "~" line
+          if (atend) {
+            used++;
+          }
+        }
+      }
+
+      if (prefer_above
+          ? (round == 1 && below >= above)
+          : (round == 1 && below > above)) {
+        // add a line above the cursor
+        topline_back(curwin, &loff);
+        if (loff.height == MAXCOL) {
+          used = MAXCOL;
+        } else {
+          used += loff.height;
+        }
         if (used > curwin->w_height_inner) {
+          done = true;
           break;
         }
-        below += boff.height;
-      } else {
-        below++;                    // count a "~" line
-        if (atend) {
-          used++;
-        }
+        above += loff.height;
+        topline = loff.lnum;
+        topfill = loff.fill;
       }
     }
-
-    if (below > above) {            // add a line above the cursor
-      topline_back(curwin, &loff);
-      if (loff.height == MAXCOL) {
-        used = MAXCOL;
-      } else {
-        used += loff.height;
-      }
-      if (used > curwin->w_height_inner) {
-        break;
-      }
-      above += loff.height;
-      topline = loff.lnum;
-      topfill = loff.fill;
+    if (done) {
+      break;
     }
   }
+
   if (!hasFolding(topline, &curwin->w_topline, NULL)) {
     curwin->w_topline = topline;
   }
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index a84d5c73a4..f1226584d7 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -2856,7 +2856,7 @@ static void nv_zet(cmdarg_T *cap)
     FALLTHROUGH;
 
   case 'z':
-    scroll_cursor_halfway(true);
+    scroll_cursor_halfway(true, false);
     redraw_later(curwin, UPD_VALID);
     set_fraction(curwin);
     break;
diff --git a/src/nvim/testdir/test_scroll_opt.vim b/src/nvim/testdir/test_scroll_opt.vim
index 77920eb8b0..64f4ced470 100644
--- a/src/nvim/testdir/test_scroll_opt.vim
+++ b/src/nvim/testdir/test_scroll_opt.vim
@@ -1,5 +1,4 @@
 " Test for reset 'scroll'
-"
 
 func Test_reset_scroll()
   let scr = &l:scroll
@@ -34,3 +33,22 @@ func Test_reset_scroll()
 
   quit!
 endfunc
+
+func Test_scolloff_even_line_count()
+   new
+   resize 6
+   setlocal scrolloff=3
+   call setline(1, range(20))
+   normal 2j
+   call assert_equal(1, getwininfo(win_getid())[0].topline)
+   normal j
+   call assert_equal(1, getwininfo(win_getid())[0].topline)
+   normal j
+   call assert_equal(2, getwininfo(win_getid())[0].topline)
+   normal j
+   call assert_equal(3, getwininfo(win_getid())[0].topline)
+
+   bwipe!
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/nvim/testdir/test_scrollbind.vim b/src/nvim/testdir/test_scrollbind.vim
index 6c5488be05..f67a559c60 100644
--- a/src/nvim/testdir/test_scrollbind.vim
+++ b/src/nvim/testdir/test_scrollbind.vim
@@ -217,8 +217,8 @@ end of window 2
 	      \ '7 line 02 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ 02',
 	      \ '56789ABCDEFGHIJKLMNOPQRSTUVWXYZ 02',
 	      \ 'UTSRQPONMLKJIHGREDCBA9876543210 02',
-	      \ '. line 11 ZYXWVUTSRQPONMLKJIHGREDCBA9876543210 11',
-	      \ '. line 11 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ 11',
+	      \ '. line 10 ZYXWVUTSRQPONMLKJIHGREDCBA9876543210 10',
+	      \ '. line 10 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ 10',
 	      \ ''],  getline(1, '$'))
   enew!
 
diff --git a/test/functional/legacy/search_spec.lua b/test/functional/legacy/search_spec.lua
index 3f1f85cf28..4228940eda 100644
--- a/test/functional/legacy/search_spec.lua
+++ b/test/functional/legacy/search_spec.lua
@@ -290,8 +290,8 @@ describe('search cmdline', function()
     -- First match
     feed('/thei')
     screen:expect([[
+        3 the             |
         4 {inc:thei}r           |
-        5 there           |
       /thei^               |
     ]])
     -- Match from initial cursor position when modifying search