diff --git a/otherbackends/raw.c b/otherbackends/raw.c
index 7977f386..9c803594 100644
--- a/otherbackends/raw.c
+++ b/otherbackends/raw.c
@@ -104,6 +104,7 @@ static void raw_sent(Plug *plug, size_t bufsize)
 {
     Raw *raw = container_of(plug, Raw, plug);
     raw->bufsize = bufsize;
+    seat_sent(raw->seat, raw->bufsize);
 }
 
 static const PlugVtable Raw_plugvt = {
diff --git a/otherbackends/rlogin.c b/otherbackends/rlogin.c
index 8289a508..30ad0526 100644
--- a/otherbackends/rlogin.c
+++ b/otherbackends/rlogin.c
@@ -116,6 +116,7 @@ static void rlogin_sent(Plug *plug, size_t bufsize)
 {
     Rlogin *rlogin = container_of(plug, Rlogin, plug);
     rlogin->bufsize = bufsize;
+    seat_sent(rlogin->seat, rlogin->bufsize);
 }
 
 static void rlogin_startup(Rlogin *rlogin, const char *ruser)
diff --git a/otherbackends/supdup.c b/otherbackends/supdup.c
index 36dbb447..e7eff6ee 100644
--- a/otherbackends/supdup.c
+++ b/otherbackends/supdup.c
@@ -600,6 +600,7 @@ static void supdup_sent(Plug *plug, size_t bufsize)
 {
     Supdup *supdup = container_of(plug, Supdup, plug);
     supdup->bufsize = bufsize;
+    seat_sent(supdup->seat, supdup->bufsize);
 }
 
 static void supdup_send_36bits(Supdup *supdup, unsigned long long thirtysix)
diff --git a/otherbackends/telnet.c b/otherbackends/telnet.c
index f8b41ac3..4b784036 100644
--- a/otherbackends/telnet.c
+++ b/otherbackends/telnet.c
@@ -662,6 +662,7 @@ static void telnet_sent(Plug *plug, size_t bufsize)
 {
     Telnet *telnet = container_of(plug, Telnet, plug);
     telnet->bufsize = bufsize;
+    seat_sent(telnet->seat, telnet->bufsize);
 }
 
 static const PlugVtable Telnet_plugvt = {
diff --git a/pscp.c b/pscp.c
index 82085962..f8a6965c 100644
--- a/pscp.c
+++ b/pscp.c
@@ -65,6 +65,7 @@ static bool pscp_eof(Seat *);
 static const SeatVtable pscp_seat_vt = {
     .output = pscp_output,
     .eof = pscp_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = filexfer_get_userpass_input,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
diff --git a/psftp.c b/psftp.c
index 8eaa6fc5..1fff45ef 100644
--- a/psftp.c
+++ b/psftp.c
@@ -47,6 +47,7 @@ static bool psftp_eof(Seat *);
 static const SeatVtable psftp_seat_vt = {
     .output = psftp_output,
     .eof = psftp_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = filexfer_get_userpass_input,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
diff --git a/putty.h b/putty.h
index d67b4e75..254e370b 100644
--- a/putty.h
+++ b/putty.h
@@ -886,6 +886,17 @@ struct SeatVtable {
      */
     bool (*eof)(Seat *seat);
 
+    /*
+     * Called by the back end to notify that the output backlog has
+     * changed size. A front end in control of the event loop won't
+     * necessarily need this (they can just keep checking it via
+     * backend_sendbuffer at every opportunity), but one buried in the
+     * depths of something else (like an SSH proxy) will need to be
+     * proactively notified that the amount of buffered data has
+     * become smaller.
+     */
+    void (*sent)(Seat *seat, size_t new_sendbuffer);
+
     /*
      * Try to get answers from a set of interactive login prompts. The
      * prompts are provided in 'p'; the bufchain 'input' holds the
@@ -1117,6 +1128,8 @@ static inline size_t seat_output(
 { return seat->vt->output(seat, err, data, len); }
 static inline bool seat_eof(Seat *seat)
 { return seat->vt->eof(seat); }
+static inline void seat_sent(Seat *seat, size_t bufsize)
+{ seat->vt->sent(seat, bufsize); }
 static inline int seat_get_userpass_input(
     Seat *seat, prompts_t *p, bufchain *input)
 { return seat->vt->get_userpass_input(seat, p, input); }
@@ -1190,6 +1203,7 @@ static inline size_t seat_stderr_pl(Seat *seat, ptrlen data)
 size_t nullseat_output(
     Seat *seat, bool is_stderr, const void *data, size_t len);
 bool nullseat_eof(Seat *seat);
+void nullseat_sent(Seat *seat, size_t bufsize);
 int nullseat_get_userpass_input(Seat *seat, prompts_t *p, bufchain *input);
 void nullseat_notify_remote_exit(Seat *seat);
 void nullseat_notify_remote_disconnect(Seat *seat);
diff --git a/ssh.h b/ssh.h
index 61fc6d14..23bed390 100644
--- a/ssh.h
+++ b/ssh.h
@@ -402,6 +402,7 @@ bool ssh_is_bare(Ssh *ssh);
 
 /* Communications back to ssh.c from the BPP */
 void ssh_conn_processed_data(Ssh *ssh);
+void ssh_sendbuffer_changed(Ssh *ssh);
 void ssh_check_frozen(Ssh *ssh);
 
 /* Functions to abort the connection, for various reasons. */
diff --git a/ssh/bpp-bare.c b/ssh/bpp-bare.c
index 3546d160..f1a889aa 100644
--- a/ssh/bpp-bare.c
+++ b/ssh/bpp-bare.c
@@ -201,4 +201,6 @@ static void ssh2_bare_bpp_handle_output(BinaryPacketProtocol *bpp)
         ssh2_bare_bpp_format_packet(s, pkt);
         ssh_free_pktout(pkt);
     }
+
+    ssh_sendbuffer_changed(bpp->ssh);
 }
diff --git a/ssh/bpp1.c b/ssh/bpp1.c
index f84d787b..b82932f7 100644
--- a/ssh/bpp1.c
+++ b/ssh/bpp1.c
@@ -376,6 +376,8 @@ static void ssh1_bpp_handle_output(BinaryPacketProtocol *bpp)
             break;
         }
     }
+
+    ssh_sendbuffer_changed(bpp->ssh);
 }
 
 static void ssh1_bpp_queue_disconnect(BinaryPacketProtocol *bpp,
diff --git a/ssh/bpp2.c b/ssh/bpp2.c
index f29b962f..dc98e27c 100644
--- a/ssh/bpp2.c
+++ b/ssh/bpp2.c
@@ -979,4 +979,6 @@ static void ssh2_bpp_handle_output(BinaryPacketProtocol *bpp)
             ssh2_bpp_enable_pending_compression(s);
         }
     }
+
+    ssh_sendbuffer_changed(bpp->ssh);
 }
diff --git a/ssh/connection2.c b/ssh/connection2.c
index 48436848..2e7102db 100644
--- a/ssh/connection2.c
+++ b/ssh/connection2.c
@@ -1157,6 +1157,7 @@ static size_t ssh2_try_send(struct ssh2_channel *c)
     if (!bufsize && c->pending_eof)
         ssh2_channel_try_eof(c);
 
+    ssh_sendbuffer_changed(s->ppl.ssh);
     return bufsize;
 }
 
diff --git a/ssh/server.c b/ssh/server.c
index 642d4ce0..6abbf332 100644
--- a/ssh/server.c
+++ b/ssh/server.c
@@ -107,6 +107,7 @@ static int server_confirm_weak_cached_hostkey(
 static const SeatVtable server_seat_vt = {
     .output = nullseat_output,
     .eof = nullseat_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = nullseat_get_userpass_input,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
@@ -187,6 +188,10 @@ LogContext *ssh_get_logctx(Ssh *ssh)
     return srv->logctx;
 }
 
+void ssh_sendbuffer_changed(Ssh *ssh)
+{
+}
+
 void ssh_throttle_conn(Ssh *ssh, int adjust)
 {
     server *srv = container_of(ssh, server, ssh);
diff --git a/ssh/sesschan.c b/ssh/sesschan.c
index f1bed6f2..f16faad2 100644
--- a/ssh/sesschan.c
+++ b/ssh/sesschan.c
@@ -186,6 +186,7 @@ static bool sesschan_get_window_pixel_size(Seat *seat, int *w, int *h);
 static const SeatVtable sesschan_seat_vt = {
     .output = sesschan_seat_output,
     .eof = sesschan_seat_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = nullseat_get_userpass_input,
     .notify_remote_exit = sesschan_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
diff --git a/ssh/ssh.c b/ssh/ssh.c
index e8724777..096c8b01 100644
--- a/ssh/ssh.c
+++ b/ssh/ssh.c
@@ -640,6 +640,7 @@ static void ssh_sent(Plug *plug, size_t bufsize)
     if (bufsize < SSH_MAX_BACKLOG) {
         ssh_throttle_all(ssh, false, bufsize);
         queue_idempotent_callback(&ssh->ic_out_raw);
+        ssh_sendbuffer_changed(ssh);
     }
 }
 
@@ -1042,6 +1043,11 @@ static size_t ssh_sendbuffer(Backend *be)
     return backlog;
 }
 
+void ssh_sendbuffer_changed(Ssh *ssh)
+{
+    seat_sent(ssh->seat, ssh_sendbuffer(&ssh->backend));
+}
+
 /*
  * Called to set the size of the window from SSH's POV.
  */
diff --git a/ssh/transport2.c b/ssh/transport2.c
index fcd2667a..2966f2d7 100644
--- a/ssh/transport2.c
+++ b/ssh/transport2.c
@@ -1484,6 +1484,7 @@ static void ssh2_transport_process_queue(PacketProtocolLayer *ppl)
      * layer's outgoing queue on to our own.
      */
     pq_concatenate(s->ppl.out_pq, s->ppl.out_pq, &s->pq_out_higher);
+    ssh_sendbuffer_changed(s->ppl.ssh);
 
     /*
      * Expect SSH2_MSG_NEWKEYS from server.
@@ -1620,6 +1621,7 @@ static void ssh2_transport_process_queue(PacketProtocolLayer *ppl)
 
         /* Pass through outgoing packets from the higher layer. */
         pq_concatenate(s->ppl.out_pq, s->ppl.out_pq, &s->pq_out_higher);
+        ssh_sendbuffer_changed(s->ppl.ssh);
 
         /* Wait for either a KEXINIT, or something setting
          * s->rekey_class. This call to ssh2_transport_pop also has
diff --git a/sshproxy.c b/sshproxy.c
index 0ffa74ff..f36a2a9b 100644
--- a/sshproxy.c
+++ b/sshproxy.c
@@ -258,6 +258,12 @@ static bool sshproxy_eof(Seat *seat)
     return false;
 }
 
+static void sshproxy_sent(Seat *seat, size_t new_bufsize)
+{
+    SshProxy *sp = container_of(seat, SshProxy, seat);
+    plug_sent(sp->plug, new_bufsize);
+}
+
 static void sshproxy_notify_remote_disconnect(Seat *seat)
 {
     SshProxy *sp = container_of(seat, SshProxy, seat);
@@ -415,6 +421,7 @@ static bool sshproxy_set_trust_status(Seat *seat, bool trusted)
 static const SeatVtable SshProxy_seat_vt = {
     .output = sshproxy_output,
     .eof = sshproxy_eof,
+    .sent = sshproxy_sent,
     .get_userpass_input = sshproxy_get_userpass_input,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = sshproxy_notify_remote_disconnect,
diff --git a/unix/plink.c b/unix/plink.c
index 606bfc69..7663b976 100644
--- a/unix/plink.c
+++ b/unix/plink.c
@@ -389,6 +389,7 @@ static bool plink_seat_interactive(Seat *seat)
 static const SeatVtable plink_seat_vt = {
     .output = plink_output,
     .eof = plink_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = plink_get_userpass_input,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
diff --git a/unix/serial.c b/unix/serial.c
index d4a1e0ba..d17e4cdd 100644
--- a/unix/serial.c
+++ b/unix/serial.c
@@ -454,6 +454,7 @@ static void serial_try_write(Serial *serial)
         bufchain_consume(&serial->output_data, ret);
     }
 
+    seat_sent(serial->seat, bufchain_size(&serial->output_data));
     serial_uxsel_setup(serial);
 }
 
diff --git a/unix/window.c b/unix/window.c
index f787fa9b..8582777b 100644
--- a/unix/window.c
+++ b/unix/window.c
@@ -389,6 +389,7 @@ static bool gtk_seat_get_cursor_position(Seat *seat, int *x, int *y);
 static const SeatVtable gtk_seat_vt = {
     .output = gtk_seat_output,
     .eof = gtk_seat_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = gtk_seat_get_userpass_input,
     .notify_remote_exit = gtk_seat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
diff --git a/utils/nullseat.c b/utils/nullseat.c
index 9b69a07a..0c773af0 100644
--- a/utils/nullseat.c
+++ b/utils/nullseat.c
@@ -7,6 +7,7 @@
 size_t nullseat_output(
     Seat *seat, bool is_stderr, const void *data, size_t len) { return 0; }
 bool nullseat_eof(Seat *seat) { return true; }
+void nullseat_sent(Seat *seat, size_t bufsize) {}
 int nullseat_get_userpass_input(
     Seat *seat, prompts_t *p, bufchain *input) { return 0; }
 void nullseat_notify_remote_exit(Seat *seat) {}
diff --git a/windows/plink.c b/windows/plink.c
index 70bf1567..ac68ce7d 100644
--- a/windows/plink.c
+++ b/windows/plink.c
@@ -83,6 +83,7 @@ static bool plink_seat_interactive(Seat *seat)
 static const SeatVtable plink_seat_vt = {
     .output = plink_output,
     .eof = plink_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = plink_get_userpass_input,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
diff --git a/windows/serial.c b/windows/serial.c
index 7f4bcf2e..3d5ea8e5 100644
--- a/windows/serial.c
+++ b/windows/serial.c
@@ -88,6 +88,7 @@ static void serial_sentdata(struct handle *h, size_t new_backlog, int err)
         seat_connection_fatal(serial->seat, "%s", error_msg);
     } else {
         serial->bufsize = new_backlog;
+        seat_sent(serial->seat, serial->bufsize);
     }
 }
 
diff --git a/windows/window.c b/windows/window.c
index 36284927..6ab8e16b 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -330,6 +330,7 @@ static bool win_seat_get_window_pixel_size(Seat *seat, int *x, int *y);
 static const SeatVtable win_seat_vt = {
     .output = win_seat_output,
     .eof = win_seat_eof,
+    .sent = nullseat_sent,
     .get_userpass_input = win_seat_get_userpass_input,
     .notify_remote_exit = win_seat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,