feat: Add hard-coded invisible color 254

doc/teapot.lyx

@@ -4018,6 +4018,18 @@ teapot
 fteapot
 \family default
  set up a default palette appropriate for their respective capabilities.
+ Currently, the color number 254 is treated specially: when the foreground
+ is set to this color, the contents of the cell are not drawn into the cell
+ at all.
+ So color 254 is a hard-coded 
+\begin_inset Quotes eld
+\end_inset
+
+invisible
+\begin_inset Quotes erd
+\end_inset
+
+ or zero-alpha color.
  Hopefully, a future enhancement will allow one to display, edit, save,
  and restore the palette.
 \end_layout
@@ -10467,7 +10479,15 @@ noprefix "false"
 
 \end_inset
 
-) or making its foreground and background the same color, etc.
+) or using the hard-coded 
+\begin_inset Quotes eld
+\end_inset
+
+invisible
+\begin_inset Quotes erd
+\end_inset
+
+ color number 254, etc.
 \end_layout
 
 \begin_layout Subsection

src/common/style.h

@@ -30,6 +30,7 @@ extern const char FloatFormat_Char[];
 typedef unsigned char ColorNum;
 #define NO_COLOR_SET 0
 #define MAX_MAX_COLORS UCHAR_MAX
+#define INVISIBLE_COLOR (UCHAR_MAX - 1)
 #define COLOR_FORMAT "%hhu"
 
 typedef enum

src/display.c

@@ -606,19 +606,22 @@ void redraw_sheet(Sheet *sheet)
 	  if (sc.bold) wattron(stdscr, A_BOLD);
 	  if (sc.underline) wattron(stdscr, A_UNDERLINE);
 	  short nextcp = hashcp;
-	  if (hashcp == usecp) {
+          chtype blank = (chtype)' ';
+	  if (hashcp == usecp && fg != INVISIBLE_COLOR) {
 	    (void)mvwaddstr(stdscr, (int)(height + sheet->oriy),
 	                            (int)(width  + sheet->orix),
 	                    buf+cutoff);
 	  } else { // Alternate colors to simulate hash
+            chtype next_char = (chtype)buf[cutoff];
+            if (fg == INVISIBLE_COLOR) next_char = blank;
 	    (void)mvwaddch(stdscr, (int)(height + sheet->oriy),
-	                   (int)(width  + sheet->orix),
+	                   (int)(width  + sheet->orix), next_char);
-	                   (chtype)buf[cutoff]);
 	    for (size_t ix = cutoff+1; buf[ix]; ++ix) {
 	      wcolor_set(stdscr, nextcp, NULL);
 	      if (nextcp == hashcp) nextcp = usecp;
 	      else nextcp = hashcp;
-	      (void)waddch(stdscr, (chtype)buf[ix]);
+              next_char = fg == INVISIBLE_COLOR ? blank : (chtype)buf[ix];
+	      (void)waddch(stdscr, next_char);
 	    }
 	  }
 	  for (fill=mbslen(buf+cutoff); fill<realsize; ++fill) {

src/fteapot.fl

@@ -194,6 +194,7 @@ class TeapotTable {open : {public Fl_Table}}
 	    fl_push_clip(xx+3, yy+3, W-6, H-6);
 	    ColorNum fgcn = sc.aspect[FOREGROUND];
 	    if (fgcn == NO_COLOR_SET) fgcn = DefaultCN[FOREGROUND];
+	    if (fgcn != INVISIBLE_COLOR) {
 	      Fl_Color cellfg = ((Fl_Color *)(cursheet->palette))[fgcn];
 	      if (sc.dim) cellfg = fl_color_average(cellfg, cellbg, 0.6f);
 	      fl_color(cellfg);
@@ -230,6 +231,7 @@ class TeapotTable {open : {public Fl_Table}}
 		      adj == LEFT ? FL_ALIGN_LEFT : FL_ALIGN_CENTER,
 		      NULL, 0);
 	      if (sc.underline) fl_xyline(xx, yy+H-7, xx+W);
+	    }
 	    fl_pop_clip();
 
 	    return;