config.def.h

@@ -56,6 +56,12 @@ int allowwindowops = 0;
 static double minlatency = 8;
 static double maxlatency = 33;
 
+/*
+ * Synchronized-Update timeout in ms
+ * https://gitlab.com/gnachman/iterm2/-/wikis/synchronized-updates-spec
+ */
+static uint su_timeout = 200;
+
 /*
  * blinking timeout (set to 0 to disable blinking) for the terminal blinking
  * attribute.

st.c

@@ -232,6 +232,33 @@ static const uchar utfmask[UTF_SIZ + 1] = {0xC0, 0x80, 0xE0, 0xF0, 0xF8};
 static const Rune utfmin[UTF_SIZ + 1] = {       0,    0,  0x80,  0x800,  0x10000};
 static const Rune utfmax[UTF_SIZ + 1] = {0x10FFFF, 0x7F, 0x7FF, 0xFFFF, 0x10FFFF};
 
+#include <time.h>
+static int su = 0;
+struct timespec sutv;
+
+static void
+tsync_begin()
+{
+	clock_gettime(CLOCK_MONOTONIC, &sutv);
+	su = 1;
+}
+
+static void
+tsync_end()
+{
+	su = 0;
+}
+
+int
+tinsync(uint timeout)
+{
+	struct timespec now;
+	if (su && !clock_gettime(CLOCK_MONOTONIC, &now)
+	       && TIMEDIFF(now, sutv) >= timeout)
+		su = 0;
+	return su;
+}
+
 ssize_t
 xwrite(int fd, const char *s, size_t len)
 {
@@ -814,6 +841,9 @@ ttynew(const char *line, char *cmd, const char *out, char **args)
 	return cmdfd;
 }
 
+static int twrite_aborted = 0;
+int ttyread_pending() { return twrite_aborted; }
+
 size_t
 ttyread(void)
 {
@@ -822,7 +852,7 @@ ttyread(void)
 	int ret, written;
 
 	/* append read bytes to unprocessed bytes */
-	ret = read(cmdfd, buf+buflen, LEN(buf)-buflen);
+	ret = twrite_aborted ? 1 : read(cmdfd, buf+buflen, LEN(buf)-buflen);
 
 	switch (ret) {
 	case 0:
@@ -830,7 +860,7 @@ ttyread(void)
 	case -1:
 		die("couldn't read from shell: %s\n", strerror(errno));
 	default:
-		buflen += ret;
+		buflen += twrite_aborted ? 0 : ret;
 		written = twrite(buf, buflen, 0);
 		buflen -= written;
 		/* keep any incomplete UTF-8 byte sequence for the next call */
@@ -990,6 +1020,7 @@ tsetdirtattr(int attr)
 void
 tfulldirt(void)
 {
+	tsync_end();
 	tsetdirt(0, term.row-1);
 }
 
@@ -1959,6 +1990,12 @@ strhandle(void)
 		xsettitle(strescseq.args[0]);
 		return;
 	case 'P': /* DCS -- Device Control String */
+		/* https://gitlab.com/gnachman/iterm2/-/wikis/synchronized-updates-spec */
+		if (strstr(strescseq.buf, "=1s") == strescseq.buf)
+			tsync_begin();  /* BSU */
+		else if (strstr(strescseq.buf, "=2s") == strescseq.buf)
+			tsync_end();  /* ESU */
+		return;
 	case '_': /* APC -- Application Program Command */
 	case '^': /* PM -- Privacy Message */
 		return;
@@ -2507,6 +2544,9 @@ twrite(const char *buf, int buflen, int show_ctrl)
 	Rune u;
 	int n;
 
+	int su0 = su;
+	twrite_aborted = 0;
+
 	for (n = 0; n < buflen; n += charsize) {
 		if (IS_SET(MODE_UTF8)) {
 			/* process a complete utf8 char */
@@ -2517,6 +2557,10 @@ twrite(const char *buf, int buflen, int show_ctrl)
 			u = buf[n] & 0xFF;
 			charsize = 1;
 		}
+		if (su0 && !su) {
+			twrite_aborted = 1;
+			break;  // ESU - allow rendering before a new BSU
+		}
 		if (show_ctrl && ISCONTROL(u)) {
 			if (u & 0x80) {
 				u &= 0x7f;

st.info

@@ -191,6 +191,7 @@ st-mono| simpleterm monocolor,
 	Ms=\E]52;%p1%s;%p2%s\007,
 	Se=\E[2 q,
 	Ss=\E[%p1%d q,
+	Sync=\EP=%p1%ds\E\\,
 
 st| simpleterm,
 	use=st-mono,

x.c

@@ -1943,6 +1943,9 @@ resize(XEvent *e)
 	cresize(e->xconfigure.width, e->xconfigure.height);
 }
 
+int tinsync(uint);
+int ttyread_pending();
+
 void
 run(void)
 {
@@ -1977,7 +1980,7 @@ run(void)
 		FD_SET(ttyfd, &rfd);
 		FD_SET(xfd, &rfd);
 
-		if (XPending(xw.dpy))
+		if (XPending(xw.dpy) || ttyread_pending())
 			timeout = 0;  /* existing events might not set xfd */
 
 		seltv.tv_sec = timeout / 1E3;
@@ -1991,7 +1994,8 @@ run(void)
 		}
 		clock_gettime(CLOCK_MONOTONIC, &now);
 
-		if (FD_ISSET(ttyfd, &rfd))
+		int ttyin = FD_ISSET(ttyfd, &rfd) || ttyread_pending();
+		if (ttyin)
 			ttyread();
 
 		xev = 0;
@@ -2015,7 +2019,7 @@ run(void)
 		 * maximum latency intervals during `cat huge.txt`, and perfect
 		 * sync with periodic updates from animations/key-repeats/etc.
 		 */
-		if (FD_ISSET(ttyfd, &rfd) || xev) {
+		if (ttyin || xev) {
 			if (!drawing) {
 				trigger = now;
 				drawing = 1;
@@ -2026,6 +2030,18 @@ run(void)
 				continue;  /* we have time, try to find idle */
 		}
 
+		if (tinsync(su_timeout)) {
+			/*
+			 * on synchronized-update draw-suspension: don't reset
+			 * drawing so that we draw ASAP once we can (just after
+			 * ESU). it won't be too soon because we already can
+			 * draw now but we skip. we set timeout > 0 to draw on
+			 * SU-timeout even without new content.
+			 */
+			timeout = minlatency;
+			continue;
+		}
+
 		/* idle detected or maxlatency exhausted -> draw */
 		timeout = -1;
 		if (blinktimeout && tattrset(ATTR_BLINK)) {