feat(fold): transparent foldtext

'foldtext' can be set to an empty string to disable and render the
line with:
  - extmark highlight
  - syntax highlighting
  - search highlighting
  - no line wrapping
  - spelling
  - conceal
  - inline virtual text
  - respects `fillchars:fold`

Currently normal virtual text is not displayed


Co-authored-by: zeertzjq <zeertzjq@outlook.com>

3 files changed, 42 insertions, 23 deletions

diff --git a/src/nvim/drawline.c b/src/nvim/drawline.c
index 7afa2493b9..ae538ed7a6 100644
--- a/src/nvim/drawline.c
+++ b/src/nvim/drawline.c
@@ -944,6 +944,10 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
   static char *at_end_str = "";       // used for p_extra when displaying curwin->w_p_lcs_chars.eol
                                       // at end-of-line
   const bool has_fold = foldinfo.fi_level != 0 && foldinfo.fi_lines > 0;
+  const bool has_foldtext = has_fold && *wp->w_p_fdt != NUL;
+
+  const bool is_wrapped = wp->w_p_wrap
+                          && !has_fold;       // Never wrap folded lines
 
   int saved_attr2 = 0;                  // char_attr saved for n_attr
   int n_attr3 = 0;                      // chars with overruling special attr
@@ -1036,7 +1040,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
     // trailing white space and/or syntax processing to be done.
     extra_check = wp->w_p_lbr;
     if (syntax_present(wp) && !wp->w_s->b_syn_error && !wp->w_s->b_syn_slow
-        && !has_fold && !end_fill) {
+        && !has_foldtext && !end_fill) {
       // Prepare for syntax highlighting in this line.  When there is an
       // error, stop syntax highlighting.
       int save_did_emsg = did_emsg;
@@ -1132,7 +1136,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       // handle 'incsearch' and ":s///c" highlighting
     } else if (highlight_match
                && wp == curwin
-               && !has_fold
+               && !has_foldtext
                && lnum >= curwin->w_cursor.lnum
                && lnum <= curwin->w_cursor.lnum + search_match_lines) {
       if (lnum == curwin->w_cursor.lnum) {
@@ -1192,7 +1196,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       // 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));
+    cul_screenline = (is_wrapped && (wp->w_p_culopt_flags & CULOPT_SCRLINE));
     if (!cul_screenline) {
       apply_cursorline_highlight(wp, &wlv);
     } else {
@@ -1315,7 +1319,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
   const schar_T lcs_eol = wp->w_p_lcs_chars.eol;  // 'eol' value
   schar_T lcs_prec_todo = wp->w_p_lcs_chars.prec;  // 'prec' until it's been used, then NUL
 
-  if (wp->w_p_list && !has_fold && !end_fill) {
+  if (wp->w_p_list && !has_foldtext && !end_fill) {
     if (wp->w_p_lcs_chars.space
         || wp->w_p_lcs_chars.multispace != NULL
         || wp->w_p_lcs_chars.leadmultispace != NULL
@@ -1467,7 +1471,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
     }
   }
 
-  if (col_rows == 0 && !has_fold && !end_fill) {
+  if (col_rows == 0 && !has_foldtext && !end_fill) {
     const int v = (int)(ptr - line);
     area_highlighting |= prepare_search_hl_line(wp, lnum, v,
                                                 &line, &screen_search_hl, &search_attr,
@@ -1605,6 +1609,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
     const bool draw_folded = has_fold && wlv.row == startrow + wlv.filler_lines;
     if (draw_folded && wlv.n_extra == 0) {
       wlv.char_attr = folded_attr = win_hl_attr(wp, HLF_FL);
+      decor_attr = 0;
     }
 
     int extmark_attr = 0;
@@ -1640,7 +1645,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
                                           &decor_state);
         }
 
-        if (!has_fold && wp->w_buffer->b_virt_text_inline > 0) {
+        if (!has_foldtext && wp->w_buffer->b_virt_text_inline > 0) {
           handle_inline_virtual_text(wp, &wlv, ptr - line);
           if (wlv.n_extra > 0 && wlv.virt_inline_hl_mode <= kHlModeReplace) {
             // restore search_attr and area_attr when n_extra is down to zero
@@ -1679,7 +1684,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
         area_active = false;
       }
 
-      if (!has_fold && wlv.n_extra == 0) {
+      if (!has_foldtext && wlv.n_extra == 0) {
         // Check for start/end of 'hlsearch' and other matches.
         // After end, check for start/end of next match.
         // When another match, have to check for start again.
@@ -1746,7 +1751,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       }
     }
 
-    if (draw_folded && wlv.n_extra == 0 && wlv.col == win_col_offset) {
+    if (draw_folded && has_foldtext && wlv.n_extra == 0 && wlv.col == win_col_offset) {
       const int v = (int)(ptr - line);
       linenr_T lnume = lnum + foldinfo.fi_lines - 1;
       memset(buf_fold, ' ', FOLD_TEXT_LEN);
@@ -1765,7 +1770,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       ptr = line + v;
     }
 
-    if (draw_folded && wlv.n_extra == 0 && wlv.col < grid->cols) {
+    if (draw_folded && wlv.n_extra == 0 && wlv.col < grid->cols && (has_foldtext || *ptr == NUL)) {
       // Fill rest of line with 'fold'.
       wlv.sc_extra = wp->w_p_fcs_chars.fold;
       wlv.sc_final = NUL;
@@ -1848,7 +1853,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       // initialize these.
       mb_c = ' ';
       mb_schar = schar_from_ascii(' ');
-    } else if (has_fold) {
+    } else if (has_foldtext || (has_fold && wlv.col >= grid->cols)) {
       // skip writing the buffer line itself
       mb_schar = NUL;
     } else {
@@ -1974,6 +1979,10 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
           can_spell = TRISTATE_TO_BOOL(decor_state.spell, can_spell);
         }
 
+        if (folded_attr) {
+          decor_attr = hl_combine_attr(folded_attr, decor_attr);
+        }
+
         if (decor_attr) {
           if (!attr_pri) {
             if (wlv.cul_attr) {
@@ -2388,7 +2397,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
             wlv.vcol_off += wlv.n_extra;
           }
           wlv.vcol += wlv.n_extra;
-          if (wp->w_p_wrap && wlv.n_extra > 0) {
+          if (is_wrapped && wlv.n_extra > 0) {
             wlv.boguscols += wlv.n_extra;
             wlv.col += wlv.n_extra;
           }
@@ -2489,7 +2498,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
           // Add a blank character to highlight.
           linebuf_char[wlv.off] = schar_from_ascii(' ');
         }
-        if (area_attr == 0 && !has_fold) {
+        if (area_attr == 0 && !has_foldtext) {
           // Use attributes from match with highest priority among
           // 'search_hl' and the match list.
           get_search_match_hl(wp,
@@ -2616,7 +2625,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       }
 
       // When the window is too narrow draw all "@" lines.
-      if (leftcols_width >= wp->w_grid.cols && wp->w_p_wrap) {
+      if (leftcols_width >= wp->w_grid.cols && is_wrapped) {
         win_draw_end(wp, schar_from_ascii('@'), true, wlv.row, wp->w_grid.rows, HLF_AT);
         set_empty_rows(wp, wlv.row);
         wlv.row = endrow;
@@ -2627,12 +2636,13 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
 
     // Show "extends" character from 'listchars' if beyond the line end and
     // 'list' is set.
+    // Don't show this with 'wrap' as the line can't be scrolled horizontally.
     if (wp->w_p_lcs_chars.ext != NUL
         && wp->w_p_list
         && !wp->w_p_wrap
         && wlv.filler_todo <= 0
         && wlv.col == grid->cols - 1
-        && !has_fold) {
+        && !has_foldtext) {
       if (has_decor && *ptr == NUL && lcs_eol == 0 && lcs_eol_todo) {
         // Tricky: there might be a virtual text just _after_ the last char
         decor_redraw_col(wp, (colnr_T)(ptr - line), wlv.off, false, &decor_state);
@@ -2714,7 +2724,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
       if (wlv.n_extra > 0) {
         wlv.vcol_off += wlv.n_extra;
       }
-      if (wp->w_p_wrap) {
+      if (is_wrapped) {
         // Special voodoo required if 'wrap' is on.
         //
         // Advance the column indicator to force the line
@@ -2782,11 +2792,11 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
     if (has_decor && wlv.filler_todo <= 0 && wlv.col >= grid->cols) {
       // At the end of screen line: might need to peek for decorations just after
       // this position.
-      if (!has_fold && wp->w_p_wrap && wlv.n_extra == 0) {
+      if (is_wrapped && wlv.n_extra == 0) {
         decor_redraw_col(wp, (colnr_T)(ptr - line), -3, false, &decor_state);
         // Check position/hiding of virtual text again on next screen line.
         decor_need_recheck = true;
-      } else if (has_fold || !wp->w_p_wrap) {
+      } else if (!is_wrapped) {
         // Without wrapping, we might need to display right_align and win_col
         // virt_text for the entire text line.
         decor_redraw_col(wp, MAXCOL, -1, true, &decor_state);
@@ -2795,14 +2805,14 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
 
     // At end of screen line and there is more to come: Display the line
     // so far.  If there is no more to display it is caught above.
-    if (wlv.col >= grid->cols && (!has_fold || virt_line_offset >= 0)
+    if (wlv.col >= grid->cols && (!has_foldtext || virt_line_offset >= 0)
         && (*ptr != NUL
             || wlv.filler_todo > 0
             || (wp->w_p_list && wp->w_p_lcs_chars.eol != NUL
                 && wlv.p_extra != at_end_str)
             || (wlv.n_extra != 0 && (wlv.sc_extra != NUL || *wlv.p_extra != NUL))
             || has_more_inline_virt(&wlv, ptr - line))) {
-      const bool wrap = wp->w_p_wrap                  // Wrapping enabled.
+      const bool wrap = is_wrapped                    // Wrapping enabled (not a folded line).
                         && wlv.filler_todo <= 0       // Not drawing diff filler lines.
                         && lcs_eol_todo               // Haven't printed the lcs_eol character.
                         && wlv.row != endrow - 1      // Not the last line being displayed.
@@ -2835,7 +2845,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
 
       // When not wrapping and finished diff lines, or when displayed
       // '$' and highlighting until last column, break here.
-      if ((!wp->w_p_wrap && wlv.filler_todo <= 0) || !lcs_eol_todo) {
+      if ((!is_wrapped && wlv.filler_todo <= 0) || !lcs_eol_todo) {
         break;
       }
 
diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index 9a3fffde07..1cc04d2588 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -2283,22 +2283,28 @@ static void win_update(win_T *wp)
           syntax_end_parsing(wp, syntax_last_parsed + 1);
         }
 
+        bool display_buf_line = (foldinfo.fi_lines == 0 || *wp->w_p_fdt == NUL);
+
         // Display one line
         spellvars_T zero_spv = { 0 };
         row = win_line(wp, lnum, srow, wp->w_grid.rows, 0,
-                       foldinfo.fi_lines > 0 ? &zero_spv : &spv, foldinfo);
+                       display_buf_line ? &spv : &zero_spv, foldinfo);
+
+        if (display_buf_line) {
+          syntax_last_parsed = lnum;
+        } else {
+          spv.spv_capcol_lnum = 0;
+        }
 
         if (foldinfo.fi_lines == 0) {
           wp->w_lines[idx].wl_folded = false;
           wp->w_lines[idx].wl_lastlnum = lnum;
           did_update = DID_LINE;
-          syntax_last_parsed = lnum;
         } else {
           foldinfo.fi_lines--;
           wp->w_lines[idx].wl_folded = true;
           wp->w_lines[idx].wl_lastlnum = lnum + foldinfo.fi_lines;
           did_update = DID_FOLD;
-          spv.spv_capcol_lnum = 0;
         }
       }
 
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index db33c94be3..8ef22d649a 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -3181,6 +3181,9 @@ return {
 
         It is not allowed to change text or jump to another window while
         evaluating 'foldtext' |textlock|.
+
+        When set to an empty string, foldtext is disabled, and the line
+        is displayed normally with highlighting and no line wrapping.
       ]=],
       full_name = 'foldtext',
       modelineexpr = true,