vim-patch:8.1.0994: fix relative cursor position #9676

Problem:    Relative cursor position is not calculated correctly.
Solution:   Always set topline, also when window is one line only.
            (Robert Webb) Add more info to getwininfo() for testing.
https://github.com/vim/vim/commit/8fcb60f961bdd134599fb016c6537fd496e800f5

3 files changed, 139 insertions, 8 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 57777f049a..c23177f23a 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -10316,6 +10316,8 @@ static dict_T *get_win_info(win_T *wp, int16_t tpnr, int16_t winnr)
   tv_dict_add_nr(dict, S_LEN("winid"), wp->handle);
   tv_dict_add_nr(dict, S_LEN("height"), wp->w_height);
   tv_dict_add_nr(dict, S_LEN("winrow"), wp->w_winrow);
+  tv_dict_add_nr(dict, S_LEN("topline"), wp->w_topline);
+  tv_dict_add_nr(dict, S_LEN("botline"), wp->w_botline - 1);
   tv_dict_add_nr(dict, S_LEN("width"), wp->w_width);
   tv_dict_add_nr(dict, S_LEN("bufnr"), wp->w_buffer->b_fnum);
   tv_dict_add_nr(dict, S_LEN("wincol"), wp->w_wincol);
diff --git a/src/nvim/testdir/test_window_cmd.vim b/src/nvim/testdir/test_window_cmd.vim
index b3ab6957dc..57fb36abb8 100644
--- a/src/nvim/testdir/test_window_cmd.vim
+++ b/src/nvim/testdir/test_window_cmd.vim
@@ -518,4 +518,132 @@ func Test_winrestcmd()
   only
 endfunc
 
+func Test_relative_cursor_position_in_one_line_window()
+  new
+  only
+  call setline(1, range(1, 10000))
+  normal 50%
+  let lnum = getcurpos()[1]
+  split
+  split
+  " make third window take as many lines as possible, other windows will
+  " become one line
+  3wincmd w
+  for i in range(1, &lines - 6)
+    wincmd +
+    redraw!
+  endfor
+
+  " first and second window should show cursor line
+  let wininfo = getwininfo()
+  call assert_equal(lnum, wininfo[0].topline)
+  call assert_equal(lnum, wininfo[1].topline)
+
+  only!
+  bwipe!
+endfunc
+
+func Test_relative_cursor_position_after_move_and_resize()
+  let so_save = &so
+  set so=0
+  enew
+  call setline(1, range(1, 10000))
+  normal 50%
+  split
+  1wincmd w
+  " Move cursor to first line in window
+  normal H
+  redraw!
+  " Reduce window height to two lines
+  let height = winheight(0)
+  while winheight(0) > 2
+    wincmd -
+    redraw!
+  endwhile
+  " move cursor to second/last line in window
+  normal j
+  " restore previous height
+  while winheight(0) < height
+    wincmd +
+    redraw!
+  endwhile
+  " make window two lines again
+  while winheight(0) > 2
+    wincmd -
+    redraw!
+  endwhile
+
+  " cursor should be at bottom line
+  let info = getwininfo(win_getid())[0]
+  call assert_equal(info.topline + 1, getcurpos()[1])
+
+  only!
+  bwipe!
+  let &so = so_save
+endfunc
+
+func Test_relative_cursor_position_after_resize()
+  let so_save = &so
+  set so=0
+  enew
+  call setline(1, range(1, 10000))
+  normal 50%
+  split
+  1wincmd w
+  let winid1 = win_getid()
+  let info = getwininfo(winid1)[0]
+  " Move cursor to second line in window
+  exe "normal " . (info.topline + 1) . "G"
+  redraw!
+  let lnum = getcurpos()[1]
+
+  " Make the window only two lines high, cursor should end up in top line
+  2wincmd w
+  exe (info.height - 2) . "wincmd +"
+  redraw!
+  let info = getwininfo(winid1)[0]
+  call assert_equal(lnum, info.topline)
+
+  only!
+  bwipe!
+  let &so = so_save
+endfunc
+
+func Test_relative_cursor_second_line_after_resize()
+  let so_save = &so
+  set so=0
+  enew
+  call setline(1, range(1, 10000))
+  normal 50%
+  split
+  1wincmd w
+  let winid1 = win_getid()
+  let info = getwininfo(winid1)[0]
+
+  " Make the window only two lines high
+  2wincmd _
+
+  " Move cursor to second line in window
+  normal H
+  normal j
+
+  " Make window size bigger, then back to 2 lines
+  for i in range(1, 10)
+    wincmd +
+    redraw!
+  endfor
+  for i in range(1, 10)
+    wincmd -
+    redraw!
+  endfor
+
+  " cursor should end up in bottom line
+  let info = getwininfo(winid1)[0]
+  call assert_equal(info.topline + 1, getcurpos()[1])
+
+  only!
+  bwipe!
+  let &so = so_save
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 2d71c19230..efdee34c10 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -5332,7 +5332,10 @@ void win_drag_vsep_line(win_T *dragwin, int offset)
 void set_fraction(win_T *wp)
 {
   if (wp->w_height_inner > 1) {
-    wp->w_fraction = ((long)wp->w_wrow * FRACTION_MULT + wp->w_height_inner / 2)
+    // When cursor is in the first line the percentage is computed as if
+    // it's halfway that line.  Thus with two lines it is 25%, with three
+    // lines 17%, etc.  Similarly for the last line: 75%, 83%, etc.
+    wp->w_fraction = ((long)wp->w_wrow * FRACTION_MULT + FRACTION_MULT / 2)
                    / (long)wp->w_height_inner;
   }
 }
@@ -5364,8 +5367,8 @@ void scroll_to_fraction(win_T *wp, int prev_height)
     int sline, line_size;
     int height = wp->w_height_inner;
 
-  /* Don't change w_topline when height is zero.  Don't set w_topline when
-   * 'scrollbind' is set and this isn't the current window. */
+    // Don't change w_topline when height is zero.  Don't set w_topline when
+    // 'scrollbind' is set and this isn't the current window.
   if (height > 0
       && (!wp->w_p_scb || wp == curwin)
       ) {
@@ -5376,8 +5379,7 @@ void scroll_to_fraction(win_T *wp, int prev_height)
     lnum = wp->w_cursor.lnum;
     if (lnum < 1)               /* can happen when starting up */
       lnum = 1;
-    wp->w_wrow = ((long)wp->w_fraction * (long)height - 1L + FRACTION_MULT / 2)
-                 / FRACTION_MULT;
+    wp->w_wrow = ((long)wp->w_fraction * (long)height - 1L) / FRACTION_MULT;
     line_size = plines_win_col(wp, lnum, (long)(wp->w_cursor.col)) - 1;
     sline = wp->w_wrow - line_size;
 
@@ -5408,7 +5410,6 @@ void scroll_to_fraction(win_T *wp, int prev_height)
           wp->w_wrow--;
         }
       }
-      set_topline(wp, lnum);
     } else if (sline > 0) {
       while (sline > 0 && lnum > 1) {
         (void)hasFoldingWin(wp, lnum, &lnum, NULL, true, NULL);
@@ -5437,12 +5438,12 @@ void scroll_to_fraction(win_T *wp, int prev_height)
         lnum++;
         wp->w_wrow -= line_size + sline;
       } else if (sline > 0) {
-        /* First line of file reached, use that as topline. */
+        // First line of file reached, use that as topline.
         lnum = 1;
         wp->w_wrow -= sline;
       }
-      set_topline(wp, lnum);
     }
+    set_topline(wp, lnum);
   }
 
   if (wp == curwin) {