fix(folds): cursorline highlight is not always applied on closed folds (#22242)

Problem:    The cursorline highlight logic checks for `w_cursor.lnum`
            which may be different from the line number passed to
            `win_line()` even when the cursor is actually on that line.
Solution:   Update cursor line highlight logic to check for the line
            number of the start of a closed fold if necessary.

3 files changed, 39 insertions, 29 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 3601dc7062..cace4b1364 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1112,20 +1112,23 @@ struct window_S {
   win_T *w_prev;              ///< link to previous window
   win_T *w_next;              ///< link to next window
   bool w_closing;                   ///< window is being closed, don't let
-                                    ///  autocommands close it too.
+                                    ///< autocommands close it too.
 
   frame_T *w_frame;             ///< frame containing this window
 
   pos_T w_cursor;                   ///< cursor position in buffer
 
   colnr_T w_curswant;               ///< Column we want to be at.  This is
-                                    ///  used to try to stay in the same column
-                                    ///  for up/down cursor motions.
+                                    ///< used to try to stay in the same column
+                                    ///< for up/down cursor motions.
 
   int w_set_curswant;               // If set, then update w_curswant the next
                                     // time through cursupdate() to the
                                     // current virtual column
 
+  linenr_T w_cursorline;            ///< Where 'cursorline' should be drawn,
+                                    ///< can be different from w_cursor.lnum
+                                    ///< for closed folds.
   linenr_T w_last_cursorline;       ///< where last 'cursorline' was drawn
   pos_T w_last_cursormoved;         ///< for CursorMoved event
 
diff --git a/src/nvim/drawline.c b/src/nvim/drawline.c
index e205850314..3bd55dbe09 100644
--- a/src/nvim/drawline.c
+++ b/src/nvim/drawline.c
@@ -303,7 +303,7 @@ static int draw_virt_text_item(buf_T *buf, int col, VirtText vt, HlMode hl_mode,
 static bool use_cursor_line_sign(win_T *wp, linenr_T lnum)
 {
   return wp->w_p_cul
-         && lnum == wp->w_cursor.lnum
+         && lnum == wp->w_cursorline
          && (wp->w_p_culopt_flags & CULOPT_NBR);
 }
 
@@ -517,7 +517,7 @@ static void get_statuscol_display_info(statuscol_T *stcp, LineDrawState *draw_st
 static bool use_cursor_line_nr(win_T *wp, linenr_T lnum, int row, int startrow, int filler_lines)
 {
   return wp->w_p_cul
-         && lnum == wp->w_cursor.lnum
+         && lnum == wp->w_cursorline
          && (wp->w_p_culopt_flags & CULOPT_NBR)
          && (row == startrow + filler_lines
              || (row > startrow + filler_lines
@@ -547,6 +547,12 @@ static inline void get_line_number_str(win_T *wp, linenr_T lnum, char *buf, size
 
 static int get_line_number_attr(win_T *wp, linenr_T lnum, int row, int startrow, int filler_lines)
 {
+  if (use_cursor_line_nr(wp, lnum, row, startrow, filler_lines)) {
+    // TODO(vim): Can we use CursorLine instead of CursorLineNr
+    // when CursorLineNr isn't set?
+    return win_hl_attr(wp, HLF_CLN);
+  }
+
   if (wp->w_p_rnu) {
     if (lnum < wp->w_cursor.lnum) {
       // Use LineNrAbove
@@ -558,12 +564,6 @@ static int get_line_number_attr(win_T *wp, linenr_T lnum, int row, int startrow,
     }
   }
 
-  if (use_cursor_line_nr(wp, lnum, row, startrow, filler_lines)) {
-    // TODO(vim): Can we use CursorLine instead of CursorLineNr
-    // when CursorLineNr isn't set?
-    return win_hl_attr(wp, HLF_CLN);
-  }
-
   return win_hl_attr(wp, HLF_N);
 }
 
@@ -960,20 +960,17 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool nochange,
   filler_todo = filler_lines;
 
   // Cursor line highlighting for 'cursorline' in the current window.
-  if (lnum == wp->w_cursor.lnum) {
-    // Do not show the cursor line in the text when Visual mode is active,
-    // because it's not clear what is selected then.
-    if (wp->w_p_cul && !(wp == curwin && VIsual_active)
-        && wp->w_p_culopt_flags != CULOPT_NBR) {
-      cul_screenline = (wp->w_p_wrap
-                        && (wp->w_p_culopt_flags & CULOPT_SCRLINE));
-      if (!cul_screenline) {
-        apply_cursorline_highlight(wp, lnum, &line_attr, &cul_attr, &line_attr_lowprio);
-      } else {
-        margin_columns_win(wp, &left_curline_col, &right_curline_col);
-      }
-      area_highlighting = true;
+  if (wp->w_p_cul && wp->w_p_culopt_flags != CULOPT_NBR && lnum == wp->w_cursorline
+      // Do not show the cursor line in the text when Visual mode is active,
+      // because it's not clear what is selected then.
+      && !(wp == curwin && VIsual_active)) {
+    cul_screenline = (wp->w_p_wrap && (wp->w_p_culopt_flags & CULOPT_SCRLINE));
+    if (!cul_screenline) {
+      apply_cursorline_highlight(wp, lnum, &line_attr, &cul_attr, &line_attr_lowprio);
+    } else {
+      margin_columns_win(wp, &left_curline_col, &right_curline_col);
     }
+    area_highlighting = true;
   }
 
   SignTextAttrs sattrs[SIGN_SHOW_MAX];  // sign attributes for the sign column
diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index 814d443031..2c0b6ea3b0 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -1552,7 +1552,15 @@ static void win_update(win_T *wp, DecorProviders *providers)
     wp->w_old_visual_col = 0;
   }
 
-  bool cursorline_standout = win_cursorline_standout(wp);
+  foldinfo_T cursorline_fi;
+  wp->w_cursorline = win_cursorline_standout(wp) ? wp->w_cursor.lnum : 0;
+  if (wp->w_p_cul) {
+    // Make sure that the cursorline on a closed fold is redrawn
+    cursorline_fi = fold_info(wp, wp->w_cursor.lnum);
+    if (cursorline_fi.fi_level > 0 && cursorline_fi.fi_lines > 0) {
+      wp->w_cursorline = cursorline_fi.fi_lnum;
+    }
+  }
 
   win_check_ns_hl(wp);
 
@@ -1608,7 +1616,7 @@ static void win_update(win_T *wp, DecorProviders *providers)
                         // if lines were inserted or deleted
                         || (wp->w_match_head != NULL
                             && buf->b_mod_xlines != 0)))))
-        || (cursorline_standout && lnum == wp->w_cursor.lnum)
+        || lnum == wp->w_cursorline
         || lnum == wp->w_last_cursorline) {
       if (lnum == mod_top) {
         top_to_mod = false;
@@ -1759,7 +1767,8 @@ static void win_update(win_T *wp, DecorProviders *providers)
       // When lines are folded, display one line for all of them.
       // Otherwise, display normally (can be several display lines when
       // 'wrap' is on).
-      foldinfo_T foldinfo = fold_info(wp, lnum);
+      foldinfo_T foldinfo = wp->w_p_cul && lnum == wp->w_cursor.lnum ?
+                            cursorline_fi : fold_info(wp, lnum);
 
       if (foldinfo.fi_lines == 0
           && idx < wp->w_lines_valid
@@ -1818,7 +1827,8 @@ static void win_update(win_T *wp, DecorProviders *providers)
       if (wp->w_p_rnu && wp->w_last_cursor_lnum_rnu != wp->w_cursor.lnum) {
         // 'relativenumber' set and cursor moved vertically: The
         // text doesn't need to be drawn, but the number column does.
-        foldinfo_T info = fold_info(wp, lnum);
+        foldinfo_T info = wp->w_p_cul && lnum == wp->w_cursor.lnum ?
+                          cursorline_fi : fold_info(wp, lnum);
         (void)win_line(wp, lnum, srow, wp->w_grid.rows, true, true,
                        info, &line_providers, &provider_err);
       }
@@ -1853,7 +1863,7 @@ static void win_update(win_T *wp, DecorProviders *providers)
 
   // Now that the window has been redrawn with the old and new cursor line,
   // update w_last_cursorline.
-  wp->w_last_cursorline = cursorline_standout ? wp->w_cursor.lnum : 0;
+  wp->w_last_cursorline = wp->w_cursorline;
 
   wp->w_last_cursor_lnum_rnu = wp->w_p_rnu ? wp->w_cursor.lnum : 0;