When MODE_SYNC is on, all terminal updates need to deferred, not just

scrolling. Check the flag before each one. GitHub issue 4983.

usr.bin/tmux/screen-write.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: screen-write.c,v 1.267 2026/06/11 14:19:59 nicm Exp $ */
+/* $OpenBSD: screen-write.c,v 1.268 2026/06/11 19:50:23 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -1158,7 +1158,8 @@ screen_write_redraw_line(struct screen_write_ctx *ctx, struct tty_ctx *ttyctx,
 
 			ttyctx->ocx = cx;
 			ttyctx->ocy = yy;
-			tty_write(tty_cmd_cell, ttyctx);
+			if (~s->mode & MODE_SYNC)
+				tty_write(tty_cmd_cell, ttyctx);
 		}
 	}
 }
@@ -1200,6 +1201,8 @@ screen_write_alignmenttest(struct screen_write_ctx *ctx)
 
 	screen_write_initctx(ctx, &ttyctx, 1, 1);
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_alignmenttest, &ttyctx);
 		return;
@@ -1234,6 +1237,8 @@ screen_write_insertcharacter(struct screen_write_ctx *ctx, u_int nx, u_int bg)
 	screen_write_collect_flush(ctx, 0, __func__);
 	ttyctx.n = nx;
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_insertcharacter, &ttyctx);
 		return;
@@ -1268,6 +1273,8 @@ screen_write_deletecharacter(struct screen_write_ctx *ctx, u_int nx, u_int bg)
 	screen_write_collect_flush(ctx, 0, __func__);
 	ttyctx.n = nx;
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_deletecharacter, &ttyctx);
 		return;
@@ -1302,6 +1309,8 @@ screen_write_clearcharacter(struct screen_write_ctx *ctx, u_int nx, u_int bg)
 	screen_write_collect_flush(ctx, 0, __func__);
 	ttyctx.n = nx;
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_clearcharacter, &ttyctx);
 		return;
@@ -1336,6 +1345,8 @@ screen_write_insertline(struct screen_write_ctx *ctx, u_int ny, u_int bg)
 		screen_write_collect_flush(ctx, 0, __func__);
 		ttyctx.n = ny;
 
+		if (s->mode & MODE_SYNC)
+			return;
 		if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 			tty_write(tty_cmd_insertline, &ttyctx);
 			return;
@@ -1361,6 +1372,8 @@ screen_write_insertline(struct screen_write_ctx *ctx, u_int ny, u_int bg)
 	screen_write_collect_flush(ctx, 0, __func__);
 	ttyctx.n = ny;
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_insertline, &ttyctx);
 		return;
@@ -1395,6 +1408,8 @@ screen_write_deleteline(struct screen_write_ctx *ctx, u_int ny, u_int bg)
 		screen_write_collect_flush(ctx, 0, __func__);
 		ttyctx.n = ny;
 
+		if (s->mode & MODE_SYNC)
+			return;
 		if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 			tty_write(tty_cmd_deleteline, &ttyctx);
 			return;
@@ -1420,6 +1435,8 @@ screen_write_deleteline(struct screen_write_ctx *ctx, u_int ny, u_int bg)
 	screen_write_collect_flush(ctx, 0, __func__);
 	ttyctx.n = ny;
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_deleteline, &ttyctx);
 		return;
@@ -1541,6 +1558,8 @@ screen_write_reverseindex(struct screen_write_ctx *ctx, u_int bg)
 		screen_write_initctx(ctx, &ttyctx, 1, 1);
 		ttyctx.bg = bg;
 
+		if (s->mode & MODE_SYNC)
+			return;
 		if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 			tty_write(tty_cmd_reverseindex, &ttyctx);
 			return;
@@ -1651,6 +1670,8 @@ screen_write_scrolldown(struct screen_write_ctx *ctx, u_int lines, u_int bg)
 	screen_write_collect_flush(ctx, 0, __func__);
 	ttyctx.n = lines;
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED || ctx->wp == NULL) {
 		tty_write(tty_cmd_scrolldown, &ttyctx);
 		return;
@@ -1697,6 +1718,8 @@ screen_write_clearendofscreen(struct screen_write_ctx *ctx, u_int bg)
 	screen_write_collect_clear(ctx, s->cy + 1, sy - (s->cy + 1));
 	screen_write_collect_flush(ctx, 0, __func__);
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED) {
 		tty_write(tty_cmd_clearendofscreen, &ttyctx);
 		return;
@@ -1766,6 +1789,8 @@ screen_write_clearstartofscreen(struct screen_write_ctx *ctx, u_int bg)
 	screen_write_collect_clear(ctx, 0, s->cy);
 	screen_write_collect_flush(ctx, 0, __func__);
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED) {
 		tty_write(tty_cmd_clearstartofscreen, &ttyctx);
 		return;
@@ -1834,6 +1859,8 @@ screen_write_clearscreen(struct screen_write_ctx *ctx, u_int bg)
 
 	screen_write_collect_clear(ctx, 0, sy);
 
+	if (s->mode & MODE_SYNC)
+		return;
 	if (~ttyctx.flags & TTY_CTX_PANE_OBSCURED) {
 		tty_write(tty_cmd_clearscreen, &ttyctx);
 		return;
@@ -2482,7 +2509,8 @@ screen_write_cell(struct screen_write_ctx *ctx, const struct grid_cell *gc)
 	if (s->mode & MODE_INSERT) {
 		screen_write_collect_flush(ctx, 0, __func__);
 		ttyctx.n = width;
-		tty_write(tty_cmd_insertcharacter, &ttyctx);
+		if (~s->mode & MODE_SYNC)
+			tty_write(tty_cmd_insertcharacter, &ttyctx);
 	}
 
 	/* If not writing, done now. */
@@ -2506,7 +2534,8 @@ screen_write_cell(struct screen_write_ctx *ctx, const struct grid_cell *gc)
 	for (i = 0, vis = 0; i < r->used; i++)
 		vis += r->ranges[i].nx;
 	if (vis >= width) {
-		tty_write(tty_cmd_cell, &ttyctx);
+		if (~s->mode & MODE_SYNC)
+			tty_write(tty_cmd_cell, &ttyctx);
 		return;
 	}
 
@@ -2515,6 +2544,8 @@ screen_write_cell(struct screen_write_ctx *ctx, const struct grid_cell *gc)
 	 * spaces in the visible regions.
 	 */
 	utf8_set(&tmp_gc.data, ' ');
+	if (s->mode & MODE_SYNC)
+		return;
 	for (i = 0; i < r->used; i++) {
 		ri = &r->ranges[i];
 		if (ri->nx == 0)
@@ -2656,7 +2687,8 @@ screen_write_combine(struct screen_write_ctx *ctx, const struct grid_cell *gc)
 	ttyctx.cell = &last;
 	if (force_wide)
 		ttyctx.flags |= TTY_CTX_CELL_INVALIDATE;
-	tty_write(tty_cmd_cell, &ttyctx);
+	if (~s->mode & MODE_SYNC)
+		tty_write(tty_cmd_cell, &ttyctx);
 	screen_write_set_cursor(ctx, cx, cy);
 
 	return (1);

usr.bin/tmux/server-client.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: server-client.c,v 1.466 2026/06/11 19:13:34 nicm Exp $ */
+/* $OpenBSD: server-client.c,v 1.467 2026/06/11 19:50:23 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -1779,7 +1779,7 @@ server_client_reset_state(struct client *c)
 	struct window_pane	*wp = server_client_get_pane(c), *loop;
 	struct screen		*s = NULL;
 	struct options		*oo = c->session->options;
-	int			 mode = 0, cursor, flags;
+	int			 mode = 0, cursor, flags, pane_mode = 0;
 	u_int			 cx = 0, cy = 0, ox, oy, sx, sy, n;
 	struct visible_ranges	*r;
 
@@ -1824,6 +1824,8 @@ server_client_reset_state(struct client *c)
 		cx = c->prompt_cursor;
 	} else if (wp != NULL && c->overlay_draw == NULL) {
 		cursor = 0;
+		pane_mode = wp->base.mode;
+
 		tty_window_offset(tty, &ox, &oy, &sx, &sy);
 		if (wp->xoff + (int)s->cx >= (int)ox &&
 		    wp->xoff + (int)s->cx <= (int)ox + (int)sx &&
@@ -1842,13 +1844,14 @@ server_client_reset_state(struct client *c)
 				cy += status_line_size(c);
 		}
 
-		if (!cursor)
+		if ((pane_mode & MODE_SYNC) || !cursor)
 			mode &= ~MODE_CURSOR;
 	} else if (c->overlay_mode == NULL || s == NULL)
 		mode &= ~MODE_CURSOR;
-
-	log_debug("%s: cursor to %u,%u", __func__, cx, cy);
-	tty_cursor(tty, cx, cy);
+	if (~pane_mode & MODE_SYNC) {
+		log_debug("%s: cursor to %u,%u", __func__, cx, cy);
+		tty_cursor(tty, cx, cy);
+	}
 
 	/*
 	 * Set mouse mode if requested. To support dragging, always use button