Add an option (scroll-on-clear) to control if tmux scrolls into history

on clear, from Robert Lange in GitHub issue 3121.

options-table.c

@@ -1080,6 +1080,14 @@ const struct options_table_entry options_table[] = {
 	          "remain-on-exit is enabled."
 	},
 
+	{ .name = "scroll-on-clear",
+	  .type = OPTIONS_TABLE_FLAG,
+	  .scope = OPTIONS_TABLE_WINDOW|OPTIONS_TABLE_PANE,
+	  .default_num = 1,
+	  .text = "Whether the contents of the screen should be scrolled into"
+		  "history when clearing the whole screen."
+	},
+
 	{ .name = "synchronize-panes",
 	  .type = OPTIONS_TABLE_FLAG,
 	  .scope = OPTIONS_TABLE_WINDOW|OPTIONS_TABLE_PANE,

screen-write.c

@@ -1427,7 +1427,11 @@ screen_write_clearendofscreen(struct screen_write_ctx *ctx, u_int bg)
 	ttyctx.bg = bg;
 
 	/* Scroll into history if it is enabled and clearing entire screen. */
-	if (s->cx == 0 && s->cy == 0 && (gd->flags & GRID_HISTORY))
+	if (s->cx == 0 &&
+	    s->cy == 0 &&
+	    (gd->flags & GRID_HISTORY) &&
+	    ctx->wp != NULL &&
+	    options_get_number(ctx->wp->options, "scroll-on-clear"))
 		grid_view_clear_history(gd, bg);
 	else {
 		if (s->cx <= sx - 1)

tmux.1

@@ -4479,6 +4479,12 @@ Set the text shown at the bottom of exited panes when
 .Ic remain-on-exit
 is enabled.
 .Pp
+.It Xo Ic scroll-on-clear
+.Op Ic on | off
+.Xc
+When the entire screen is cleared and this option is on, scroll the contents of
+the screen into history before clearing it.
+.Pp
 .It Xo Ic synchronize-panes
 .Op Ic on | off
 .Xc