If we can identify the terminal as iTerm2 or as tmux, we can be sure

they support 256 and RGB colours, so set those flags too.

tmux.h

@@ -1171,6 +1171,7 @@ struct tty_term {
 #define TERM_NOXENL 0x2
 #define TERM_DECSLRM 0x4
 #define TERM_DECFRA 0x8
+#define TERM_RGBCOLOURS 0x10
 	int		 flags;
 
 	LIST_ENTRY(tty_term) entry;

tty-keys.c

@@ -1117,7 +1117,9 @@ tty_keys_device_status_report(struct tty *tty, const char *buf, size_t len,
 
 	/* Set terminal flags. */
 	if (strncmp(tmp, "ITERM2 ", 7) == 0)
-		flags |= TERM_DECSLRM;
+		flags |= (TERM_DECSLRM|TERM_256COLOURS|TERM_RGBCOLOURS);
+	if (strncmp(tmp, "TMUX ", 5) == 0)
+		flags |= (TERM_256COLOURS|TERM_RGBCOLOURS);
 	log_debug("%s: received DSR %.*s", c->name, (int)*size, buf);
 
 	tty_set_flags(tty, flags);

tty-term.c

@@ -522,11 +522,16 @@ tty_term_find(char *name, int fd, char **cause)
 		goto error;
 	}
 
-	/* Figure out if we have 256 colours (or more). */
-	if (tty_term_number(term, TTYC_COLORS) >= 256 ||
-	    tty_term_has(term, TTYC_RGB))
+	/* Set flag if terminal has 256 colours. */
+	if (tty_term_number(term, TTYC_COLORS) >= 256)
 		term->flags |= TERM_256COLOURS;
 
+	/* Set flag if terminal has RGB colours. */
+	if ((tty_term_flag(term, TTYC_TC) || tty_term_has(term, TTYC_RGB)) ||
+	    (tty_term_has(term, TTYC_SETRGBF) &&
+	    tty_term_has(term, TTYC_SETRGBB)))
+		term->flags |= TERM_RGBCOLOURS;
+
 	/*
 	 * Terminals without xenl (eat newline glitch) wrap at at $COLUMNS - 1
 	 * rather than $COLUMNS (the cursor can never be beyond $COLUMNS - 1).
@@ -561,22 +566,7 @@ tty_term_find(char *name, int fd, char **cause)
 		code->type = TTYCODE_STRING;
 	}
 
-	/*
-	 * On terminals with RGB colour (Tc or RGB), fill in setrgbf and
-	 * setrgbb if they are missing.
-	 */
-	if ((tty_term_flag(term, TTYC_TC) || tty_term_flag(term, TTYC_RGB)) &&
-	    !tty_term_has(term, TTYC_SETRGBF) &&
-	    !tty_term_has(term, TTYC_SETRGBB)) {
-		code = &term->codes[TTYC_SETRGBF];
-		code->value.string = xstrdup("\033[38;2;%p1%d;%p2%d;%p3%dm");
-		code->type = TTYCODE_STRING;
-		code = &term->codes[TTYC_SETRGBB];
-		code->value.string = xstrdup("\033[48;2;%p1%d;%p2%d;%p3%dm");
-		code->type = TTYCODE_STRING;
-	}
-
-	/* Log it. */
+	/* Log the capabilities. */
 	for (i = 0; i < tty_term_ncodes(); i++)
 		log_debug("%s%s", name, tty_term_describe(term, i));
 

tty.c

@@ -2386,11 +2386,10 @@ tty_check_fg(struct tty *tty, struct window_pane *wp, struct grid_cell *gc)
 	/* Is this a 24-bit colour? */
 	if (gc->fg & COLOUR_FLAG_RGB) {
 		/* Not a 24-bit terminal? Translate to 256-colour palette. */
-		if (!tty_term_has(tty->term, TTYC_SETRGBF)) {
-			colour_split_rgb(gc->fg, &r, &g, &b);
-			gc->fg = colour_find_rgb(r, g, b);
-		} else
+		if ((tty->term->flags|tty->term_flags) & TERM_RGBCOLOURS)
 			return;
+		colour_split_rgb(gc->fg, &r, &g, &b);
+		gc->fg = colour_find_rgb(r, g, b);
 	}
 
 	/* How many colours does this terminal have? */
@@ -2436,11 +2435,10 @@ tty_check_bg(struct tty *tty, struct window_pane *wp, struct grid_cell *gc)
 	/* Is this a 24-bit colour? */
 	if (gc->bg & COLOUR_FLAG_RGB) {
 		/* Not a 24-bit terminal? Translate to 256-colour palette. */
-		if (!tty_term_has(tty->term, TTYC_SETRGBB)) {
-			colour_split_rgb(gc->bg, &r, &g, &b);
-			gc->bg = colour_find_rgb(r, g, b);
-		} else
+		if ((tty->term->flags|tty->term_flags) & TERM_RGBCOLOURS)
 			return;
+		colour_split_rgb(gc->bg, &r, &g, &b);
+		gc->bg = colour_find_rgb(r, g, b);
 	}
 
 	/* How many colours does this terminal have? */
@@ -2617,15 +2615,14 @@ tty_try_colour(struct tty *tty, int colour, const char *type)
 	}
 
 	if (colour & COLOUR_FLAG_RGB) {
+		colour_split_rgb(colour & 0xffffff, &r, &g, &b);
 		if (*type == '3') {
 			if (!tty_term_has(tty->term, TTYC_SETRGBF))
-				return (-1);
-			colour_split_rgb(colour & 0xffffff, &r, &g, &b);
+				goto fallback_rgb;
 			tty_putcode3(tty, TTYC_SETRGBF, r, g, b);
 		} else {
 			if (!tty_term_has(tty->term, TTYC_SETRGBB))
-				return (-1);
-			colour_split_rgb(colour & 0xffffff, &r, &g, &b);
+				goto fallback_rgb;
 			tty_putcode3(tty, TTYC_SETRGBB, r, g, b);
 		}
 		return (0);
@@ -2638,6 +2635,12 @@ tty_try_colour(struct tty *tty, int colour, const char *type)
 	log_debug("%s: 256 colour fallback: %s", tty->client->name, s);
 	tty_puts(tty, s);
 	return (0);
+
+fallback_rgb:
+	xsnprintf(s, sizeof s, "\033[%s;2;%d;%d;%dm", type, r, g, b);
+	log_debug("%s: RGB colour fallback: %s", tty->client->name, s);
+	tty_puts(tty, s);
+	return (0);
 }
 
 static void