Add a clear-on-attach option which when turned off scrolls away the

existing terminal state instead of entering the alternate screen. This
is useful in environments where tmux is started as a login shell but it
is important that any pre-tmux messages are still visible. GitHub issue
5508 from Josef Řídký.

options-table.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: options-table.c,v 1.242 2026/07/27 08:03:01 nicm Exp $ */
+/* $OpenBSD: options-table.c,v 1.243 2026/08/25 08:37:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2011 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -306,6 +306,16 @@ const struct options_table_entry options_table[] = {
 		  "When this is reached, the oldest buffer is deleted."
 	},
 
+	{ .name = "clear-on-attach",
+	  .type = OPTIONS_TABLE_FLAG,
+	  .scope = OPTIONS_TABLE_SERVER,
+	  .default_num = 1,
+	  .text = "Whether to use the alternate screen and clear it when "
+		  "a client is attached. When disabled, tmux does not "
+		  "enter the alternate screen on attach so terminal "
+		  "content before tmux remains in scrollback."
+	},
+
 	{ .name = "command-alias",
 	  .type = OPTIONS_TABLE_STRING,
 	  .scope = OPTIONS_TABLE_SERVER,

tmux.1

@@ -1,4 +1,4 @@
-.\" $OpenBSD: tmux.1,v 1.1158 2026/08/20 09:19:24 nicm Exp $
+.\" $OpenBSD: tmux.1,v 1.1159 2026/08/25 08:37:08 nicm Exp $
 .\"
 .\" Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
 .\"
@@ -14,7 +14,7 @@
 .\" IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
 .\" OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: August 20 2026 $
+.Dd $Mdocdate: August 25 2026 $
 .Dt TMUX 1
 .Os
 .Sh NAME
@@ -4752,6 +4752,15 @@ for backspace.
 Set the number of buffers; as new buffers are added to the top of the stack,
 old ones are removed from the bottom if necessary to maintain this maximum
 length.
+.It Xo Ic clear\-on\-attach
+.Op Ic on | off
+.Xc
+When enabled (the default), the client terminal switches to the alternate
+screen and clears it when a client is attached.
+When disabled,
+.Nm
+does not enter the alternate screen; instead the existing terminal content is
+scrolled into the scrollback buffer so it remains accessible.
 .It Xo Ic command\-alias[]
 .Ar name=value
 .Xc

tmux.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: tmux.h,v 1.1430 2026/08/24 21:17:19 nicm Exp $ */
+/* $OpenBSD: tmux.h,v 1.1431 2026/08/25 08:37:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -474,6 +474,7 @@ enum tty_code_code {
 	TTYC_ICH1,
 	TTYC_IL,
 	TTYC_IL1,
+	TTYC_IND,
 	TTYC_INDN,
 	TTYC_INVIS,
 	TTYC_KCBT,

tty-term.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: tty-term.c,v 1.108 2026/08/17 14:47:41 nicm Exp $ */
+/* $OpenBSD: tty-term.c,v 1.109 2026/08/25 08:37:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2008 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -108,6 +108,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_ICH] = { TTYCODE_STRING, "ich" },
 	[TTYC_IL1] = { TTYCODE_STRING, "il1" },
 	[TTYC_IL] = { TTYCODE_STRING, "il" },
+	[TTYC_IND] = { TTYCODE_STRING, "ind" },
 	[TTYC_INDN] = { TTYCODE_STRING, "indn" },
 	[TTYC_INVIS] = { TTYCODE_STRING, "invis" },
 	[TTYC_KCBT] = { TTYCODE_STRING, "kcbt" },

tty.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: tty.c,v 1.479 2026/08/18 09:01:20 nicm Exp $ */
+/* $OpenBSD: tty.c,v 1.480 2026/08/25 08:37:08 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -333,6 +333,7 @@ tty_start_tty(struct tty *tty)
 {
 	struct client	*c = tty->client;
 	struct termios	 tio;
+	u_int		 i;
 
 	setblocking(c->fd, 0);
 	event_add(&tty->event_in, NULL);
@@ -348,10 +349,21 @@ tty_start_tty(struct tty *tty)
 	if (tcsetattr(c->fd, TCSANOW, &tio) == 0)
 		tcflush(c->fd, TCOFLUSH);
 
-	tty_putcode(tty, TTYC_SMCUP);
+	if (options_get_number(global_options, "clear-on-attach")) {
-
+		tty_putcode(tty, TTYC_SMCUP);
+		tty_putcode(tty, TTYC_CLEAR);
+	} else {
+		tty_putcode_ii(tty, TTYC_CSR, 0, tty->sy - 1);
+		tty_putcode_ii(tty, TTYC_CUP, 0, tty->sy - 1);
+		if (tty_term_has(tty->term, TTYC_INDN))
+			tty_putcode_i(tty, TTYC_INDN, tty->sy + 1);
+		else if (tty_term_has(tty->term, TTYC_IND)) {
+			for (i = 0; i < tty->sy + 1; i++)
+				tty_putcode(tty, TTYC_IND);
+		} else
+			tty_putcode(tty, TTYC_CLEAR);
+	}
 	tty_putcode(tty, TTYC_SMKX);
-	tty_putcode(tty, TTYC_CLEAR);
 
 	if (tty_acs_needed(tty)) {
 		log_debug("%s: using capabilities for ACS", c->name);
@@ -467,7 +479,8 @@ tty_stop_tty(struct tty *tty)
 		tty_raw(tty, tty_term_string(tty->term, TTYC_RMACS));
 	tty_raw(tty, tty_term_string(tty->term, TTYC_SGR0));
 	tty_raw(tty, tty_term_string(tty->term, TTYC_RMKX));
-	tty_raw(tty, tty_term_string(tty->term, TTYC_CLEAR));
+	if (options_get_number(global_options, "clear-on-attach"))
+		tty_raw(tty, tty_term_string(tty->term, TTYC_CLEAR));
 	if (tty->cstyle != SCREEN_CURSOR_DEFAULT) {
 		if (tty_term_has(tty->term, TTYC_SE))
 			tty_raw(tty, tty_term_string(tty->term, TTYC_SE));
@@ -492,7 +505,10 @@ tty_stop_tty(struct tty *tty)
 
 	if (tty_use_margin(tty))
 		tty_raw(tty, tty_term_string(tty->term, TTYC_DSMG));
-	tty_raw(tty, tty_term_string(tty->term, TTYC_RMCUP));
+	if (options_get_number(global_options, "clear-on-attach"))
+		tty_raw(tty, tty_term_string(tty->term, TTYC_RMCUP));
+	else
+		tty_raw(tty, tty_term_string(tty->term, TTYC_CLEAR));
 
 	if (tty->term->flags & TERM_VT100LIKE)
 		tty_raw(tty, "\033[?2031l");