vis: add workaround for broken color handling in Terminal.app

Terminal.app sets $TERM=xterm-256color and ships a corresponding
terminfo description advocating that it is capable of color changes
to the 256 color palette when in fact it can not.

We introduce a new boolean option "change-256colors" which is
true by default but can be used to disable color changes. It is
automatically set if Terminal.app is detected using $TERM_PROGRAM.

This should fix display artifacts as described in #456.

6 files changed, 25 insertions, 9 deletions

diff --git a/lua/vis-std.lua b/lua/vis-std.lua
index 4da7293..6c42a0c 100644
--- a/lua/vis-std.lua
+++ b/lua/vis-std.lua
@@ -19,6 +19,9 @@ vis.events.subscribe(vis.events.INIT, function()
 		vis.lexers = require('lexer')
 	end
 
+	if os.getenv("TERM_PROGRAM") == "Apple_Terminal" then
+		vis:command("set change-256colors false");
+	end
 	vis:command("set theme ".. (vis.ui.colors <= 16 and "default-16" or "default-256"))
 end)
 
diff --git a/sam.c b/sam.c
index 95b2690..b232726 100644
--- a/sam.c
+++ b/sam.c
@@ -302,6 +302,7 @@ enum {
 	OPTION_COLOR_COLUMN,
 	OPTION_HORIZON,
 	OPTION_SAVE_METHOD,
+	OPTION_CHANGE_256COLORS,
 };
 
 static const OptionDef options[] = {
@@ -385,6 +386,12 @@ static const OptionDef options[] = {
 		OPTION_TYPE_STRING, OPTION_FLAG_WINDOW,
 		"Save method to use for current file 'auto', 'atomic' or 'inplace'",
 	},
+	[OPTION_CHANGE_256COLORS] = {
+		{ "change-256colors" },
+		OPTION_TYPE_BOOL, OPTION_FLAG_NONE,
+		"Change 256 color palette to support 24bit colors",
+	},
+
 };
 
 bool sam_init(Vis *vis) {
diff --git a/ui-curses.c b/ui-curses.c
index 94734d5..81a075f 100644
--- a/ui-curses.c
+++ b/ui-curses.c
@@ -65,7 +65,7 @@
 #define MAX_COLOR_CLOBBER 240
 static short color_clobber_idx = 0;
 static uint32_t clobbering_colors[MAX_COLOR_CLOBBER];
-static bool change_colors;
+static int change_colors = -1;
 
 typedef struct {
 	attr_t attr;
@@ -143,8 +143,10 @@ static void undo_palette(void)
 }
 
 /* Work out the nearest color from the 256 color set, or perhaps exactly. */
-static int color_find_rgb(unsigned char r, unsigned char g, unsigned char b)
+static int color_find_rgb(UiCurses *ui, unsigned char r, unsigned char g, unsigned char b)
 {
+	if (change_colors == -1)
+		change_colors = ui->vis->change_colors && can_change_color() && COLORS >= 256;
 	if (change_colors) {
 		uint32_t hexrep = ((r << 16) | (g << 8) | b) + 1;
 		for (short i = 0; i < MAX_COLOR_CLOBBER; ++i) {
@@ -217,7 +219,7 @@ static int color_find_rgb(unsigned char r, unsigned char g, unsigned char b)
 }
 
 /* Convert color from string. */
-static int color_fromstring(const char *s)
+static int color_fromstring(UiCurses *ui, const char *s)
 {
 	if (!s)
 		return -1;
@@ -230,7 +232,7 @@ static int color_fromstring(const char *s)
 		int n = sscanf(s + 1, "%2hhx%2hhx%2hhx", &r, &g, &b);
 		if (n != 3)
 			return -1;
-		return color_find_rgb(r, g, b);
+		return color_find_rgb(ui, r, g, b);
 	} else if ('0' <= *s && *s <= '9') {
 		int col = atoi(s);
 		return (col <= 0 || col > 255) ? -1 : col;
@@ -349,9 +351,9 @@ static bool ui_window_syntax_style(UiWin *w, int id, const char *style) {
 		} else if (!strcasecmp(option, "notblink")) {
 			cell_style.attr &= ~A_BLINK;
 		} else if (!strcasecmp(option, "fore")) {
-			cell_style.fg = color_fromstring(p);
+			cell_style.fg = color_fromstring(win->ui, p);
 		} else if (!strcasecmp(option, "back")) {
-			cell_style.bg = color_fromstring(p);
+			cell_style.bg = color_fromstring(win->ui, p);
 		}
 		option = next;
 	}
@@ -830,7 +832,7 @@ static TermKey *ui_termkey_get(Ui *ui) {
 }
 
 static void ui_suspend(Ui *ui) {
-	if (change_colors)
+	if (change_colors == 1)
 		undo_palette();
 	endwin();
 	kill(0, SIGSTOP);
@@ -913,7 +915,6 @@ static bool ui_init(Ui *ui, Vis *vis) {
 	keypad(stdscr, TRUE);
 	meta(stdscr, TRUE);
 	curs_set(0);
-	change_colors = can_change_color() && COLORS >= 256;
 
 	ui_resize(ui);
 	return true;
@@ -960,7 +961,7 @@ void ui_curses_free(Ui *ui) {
 		return;
 	while (uic->windows)
 		ui_window_free((UiWin*)uic->windows);
-	if (change_colors)
+	if (change_colors == 1)
 		undo_palette();
 	endwin();
 	if (uic->termkey)
diff --git a/vis-cmds.c b/vis-cmds.c
index 879eadd..8e512cc 100644
--- a/vis-cmds.c
+++ b/vis-cmds.c
@@ -291,6 +291,9 @@ static bool cmd_set(Vis *vis, Win *win, Command *cmd, const char *argv[], Cursor
 			return false;
 		}
 		break;
+	case OPTION_CHANGE_256COLORS:
+		vis->change_colors = toggle ? !vis->change_colors : arg.b;
+		break;
 	default:
 		return false;
 	}
diff --git a/vis-core.h b/vis-core.h
index cc8636d..66c71c4 100644
--- a/vis-core.h
+++ b/vis-core.h
@@ -168,6 +168,7 @@ struct Vis {
 	int tabwidth;                        /* how many spaces should be used to display a tab */
 	bool expandtab;                      /* whether typed tabs should be converted to spaces */
 	bool autoindent;                     /* whether indentation should be copied from previous line on newline */
+	bool change_colors;                  /* whether to adjust 256 color palette for true colors */
 	char *shell;                         /* shell used to launch external commands */
 	Map *cmds;                           /* ":"-commands, used for unique prefix queries */
 	Map *usercmds;                       /* user registered ":"-commands */
diff --git a/vis.c b/vis.c
index 790069a..e0b2c1e 100644
--- a/vis.c
+++ b/vis.c
@@ -532,6 +532,7 @@ Vis *vis_new(Ui *ui, VisEvent *event) {
 	vis->ui = ui;
 	vis->tabwidth = 8;
 	vis->expandtab = false;
+	vis->change_colors = true;
 	vis->registers[VIS_REG_BLACKHOLE].type = REGISTER_BLACKHOLE;
 	vis->registers[VIS_REG_CLIPBOARD].type = REGISTER_CLIPBOARD;
 	array_init(&vis->motions);