Make ESC[3J (clear scrollback) a disableable escape sequence.

A user complained that it was being done nonconsensually, and it seems
reasonable that the user should have the choice to prevent it.

config.c

@@ -1690,6 +1690,10 @@ void setup_config_box(struct controlbox *b, int midsession,
 		  HELPCTX(features_retitle),
 		  conf_checkbox_handler,
 		  I(CONF_no_remote_wintitle));
+    ctrl_checkbox(s, "Disable remote-controlled clearing of scrollback", 'e',
+		  HELPCTX(features_clearscroll),
+		  conf_checkbox_handler,
+		  I(CONF_no_remote_clearscroll));
     ctrl_radiobuttons(s, "Response to remote title query (SECURITY):", 'q', 3,
 		      HELPCTX(features_qtitle),
 		      conf_radiobutton_handler,

doc/config.but

@@ -934,6 +934,15 @@ setting you want if you have no better ideas.
 \dd PuTTY responds with the actual window title. This is dangerous for
 the reasons described above.
 
+\S{config-features-clearscroll} Disabling remote \i{scrollback clearing}
+
+\cfg{winhelp-topic}{features.clearscroll}
+
+PuTTY has the ability to clear the terminal's scrollback buffer in
+response to a command from the server. If you find PuTTY is doing this
+unexpectedly or inconveniently, you can tell PuTTY not to respond to
+that server command.
+
 \S{config-features-dbackspace} Disabling \i{destructive backspace}
 
 \cfg{winhelp-topic}{features.dbackspace}

putty.h

@@ -770,6 +770,7 @@ void cleanup_exit(int);
     X(INT, NONE, no_remote_resize) /* disable remote resizing */ \
     X(INT, NONE, no_alt_screen) /* disable alternate screen */ \
     X(INT, NONE, no_remote_wintitle) /* disable remote retitling */ \
+    X(INT, NONE, no_remote_clearscroll) /* disable ESC[3J */ \
     X(INT, NONE, no_dbackspace) /* disable destructive backspace */ \
     X(INT, NONE, no_remote_charset) /* disable remote charset config */ \
     X(INT, NONE, remote_qtitle_action) /* remote win title query action */ \

settings.c

@@ -545,6 +545,7 @@ void save_open_settings(void *sesskey, Conf *conf)
     write_setting_i(sesskey, "NoRemoteResize", conf_get_int(conf, CONF_no_remote_resize));
     write_setting_i(sesskey, "NoAltScreen", conf_get_int(conf, CONF_no_alt_screen));
     write_setting_i(sesskey, "NoRemoteWinTitle", conf_get_int(conf, CONF_no_remote_wintitle));
+    write_setting_i(sesskey, "NoRemoteClearScroll", conf_get_int(conf, CONF_no_remote_clearscroll));
     write_setting_i(sesskey, "RemoteQTitleAction", conf_get_int(conf, CONF_remote_qtitle_action));
     write_setting_i(sesskey, "NoDBackspace", conf_get_int(conf, CONF_no_dbackspace));
     write_setting_i(sesskey, "NoRemoteCharset", conf_get_int(conf, CONF_no_remote_charset));
@@ -876,6 +877,7 @@ void load_open_settings(void *sesskey, Conf *conf)
     gppi(sesskey, "NoRemoteResize", 0, conf, CONF_no_remote_resize);
     gppi(sesskey, "NoAltScreen", 0, conf, CONF_no_alt_screen);
     gppi(sesskey, "NoRemoteWinTitle", 0, conf, CONF_no_remote_wintitle);
+    gppi(sesskey, "NoRemoteClearScroll", 0, conf, CONF_no_remote_clearscroll);
     {
 	/* Backward compatibility */
 	int no_remote_qtitle = gppi_raw(sesskey, "NoRemoteQTitle", 1);

terminal.c

@@ -1403,6 +1403,7 @@ void term_copy_stuff_from_conf(Terminal *term)
     term->no_remote_charset = conf_get_int(term->conf, CONF_no_remote_charset);
     term->no_remote_resize = conf_get_int(term->conf, CONF_no_remote_resize);
     term->no_remote_wintitle = conf_get_int(term->conf, CONF_no_remote_wintitle);
+    term->no_remote_clearscroll = conf_get_int(term->conf, CONF_no_remote_clearscroll);
     term->rawcnp = conf_get_int(term->conf, CONF_rawcnp);
     term->rect_select = conf_get_int(term->conf, CONF_rect_select);
     term->remote_qtitle_action = conf_get_int(term->conf, CONF_remote_qtitle_action);
@@ -3606,6 +3607,7 @@ static void term_out(Terminal *term)
 			    if (i == 3) {
 				/* Erase Saved Lines (xterm)
 				 * This follows Thomas Dickey's xterm. */
+                                if (!term->no_remote_clearscroll)
                                     term_clrsb(term);
 			    } else {
 				i++;

terminal.h

@@ -315,6 +315,7 @@ struct terminal_tag {
     int no_remote_charset;
     int no_remote_resize;
     int no_remote_wintitle;
+    int no_remote_clearscroll;
     int rawcnp;
     int rect_select;
     int remote_qtitle_action;

windows/winhelp.h

@@ -41,6 +41,7 @@
 #define WINHELP_CTX_features_qtitle "features.qtitle:config-features-qtitle"
 #define WINHELP_CTX_features_dbackspace "features.dbackspace:config-features-dbackspace"
 #define WINHELP_CTX_features_charset "features.charset:config-features-charset"
+#define WINHELP_CTX_features_clearscroll "features.clearscroll:config-features-clearscroll"
 #define WINHELP_CTX_features_arabicshaping "features.arabicshaping:config-features-shaping"
 #define WINHELP_CTX_features_bidi "features.bidi:config-features-bidi"
 #define WINHELP_CTX_terminal_autowrap "terminal.autowrap:config-autowrap"