auto-sync patch

config.h

@@ -42,9 +42,14 @@ static unsigned int tripleclicktimeout = 600;
 /* alt screens */
 int allowaltscreen = 1;
 
-/* frames per second st should at maximum draw to the screen */
+/*
-static unsigned int xfps = 120;
+ * draw latency range in ms - from new content/keypress/etc until drawing.
-static unsigned int actionfps = 30;
+ * within this range, st draws when content stops arriving (idle). mostly it's
+ * near minlatency, but it waits longer for slow updates to avoid partial draw.
+ * low minlatency will tear/flicker more, as it can "detect" idle too early.
+ */
+static double minlatency = 8;
+static double maxlatency = 33;
 
 /*
  * blinking timeout (set to 0 to disable blinking) for the terminal blinking
@@ -197,8 +202,6 @@ ResourcePref resources[] = {
 		{ "cursorColor",  STRING,  &colorname[256] },
 		{ "termname",     STRING,  &termname },
 		{ "shell",        STRING,  &shell },
-		{ "xfps",         INTEGER, &xfps },
-		{ "actionfps",    INTEGER, &actionfps },
 		{ "blinktimeout", INTEGER, &blinktimeout },
 		{ "bellvolume",   INTEGER, &bellvolume },
 		{ "tabspaces",    INTEGER, &tabspaces },

x.c

@@ -1956,10 +1956,9 @@ run(void)
 	XEvent ev;
 	int w = win.w, h = win.h;
 	fd_set rfd;
-	int xfd = XConnectionNumber(xw.dpy), xev, blinkset = 0, dodraw = 0;
+	int xfd = XConnectionNumber(xw.dpy), ttyfd, xev, drawing;
-	int ttyfd;
+	struct timespec seltv, *tv, now, lastblink, trigger;
-	struct timespec drawtimeout, *tv = NULL, now, last, lastblink;
+	double timeout;
-	long deltatime;
 
 	/* Waiting for window mapping */
 	do {
@@ -1980,51 +1979,31 @@ run(void)
 	ttyfd = ttynew(opt_line, shell, opt_io, opt_cmd);
 	cresize(w, h);
 
-	clock_gettime(CLOCK_MONOTONIC, &last);
+	for (timeout = -1, drawing = 0, lastblink = (struct timespec){0};;) {
-	lastblink = last;
-
-	for (xev = actionfps;;) {
 		FD_ZERO(&rfd);
 		FD_SET(ttyfd, &rfd);
 		FD_SET(xfd, &rfd);
 
+		if (XPending(xw.dpy))
+			timeout = 0;  /* existing events might not set xfd */
+
+		seltv.tv_sec = timeout / 1E3;
+		seltv.tv_nsec = 1E6 * (timeout - 1E3 * seltv.tv_sec);
+		tv = timeout >= 0 ? &seltv : NULL;
+
 		if (pselect(MAX(xfd, ttyfd)+1, &rfd, NULL, NULL, tv, NULL) < 0) {
 			if (errno == EINTR)
 				continue;
 			die("select failed: %s\n", strerror(errno));
 		}
-		if (FD_ISSET(ttyfd, &rfd)) {
-			ttyread();
-			if (blinktimeout) {
-				blinkset = tattrset(ATTR_BLINK);
-				if (!blinkset)
-					MODBIT(win.mode, 0, MODE_BLINK);
-			}
-		}
-
-		if (FD_ISSET(xfd, &rfd))
-			xev = actionfps;
-
 		clock_gettime(CLOCK_MONOTONIC, &now);
-		drawtimeout.tv_sec = 0;
-		drawtimeout.tv_nsec =  (1000 * 1E6)/ xfps;
-		tv = &drawtimeout;
 
-		dodraw = 0;
+		if (FD_ISSET(ttyfd, &rfd))
-		if (blinktimeout && TIMEDIFF(now, lastblink) > blinktimeout) {
+			ttyread();
-			tsetdirtattr(ATTR_BLINK);
-			win.mode ^= MODE_BLINK;
-			lastblink = now;
-			dodraw = 1;
-		}
-		deltatime = TIMEDIFF(now, last);
-		if (deltatime > 1000 / (xev ? xfps : actionfps)) {
-			dodraw = 1;
-			last = now;
-		}
 
-		if (dodraw) {
+		xev = 0;
 		while (XPending(xw.dpy)) {
+			xev = 1;
 			XNextEvent(xw.dpy, &ev);
 			if (XFilterEvent(&ev, None))
 				continue;
@@ -2032,30 +2011,45 @@ run(void)
 				(handler[ev.type])(&ev);
 		}
 
+		/*
+		 * To reduce flicker and tearing, when new content or event
+		 * triggers drawing, we first wait a bit to ensure we got
+		 * everything, and if nothing new arrives - we draw.
+		 * We start with trying to wait minlatency ms. If more content
+		 * arrives sooner, we retry with shorter and shorter preiods,
+		 * and eventually draw even without idle after maxlatency ms.
+		 * Typically this results in low latency while interacting,
+		 * 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 (!drawing) {
+				trigger = now;
+				drawing = 1;
+			}
+			timeout = (maxlatency - TIMEDIFF(now, trigger)) \
+			          / maxlatency * minlatency;
+			if (timeout > 0)
+				continue;  /* we have time, try to find idle */
+		}
+
+		/* idle detected or maxlatency exhausted -> draw */
+		timeout = -1;
+		if (blinktimeout && tattrset(ATTR_BLINK)) {
+			timeout = blinktimeout - TIMEDIFF(now, lastblink);
+			if (timeout <= 0) {
+				if (-timeout > blinktimeout) /* start visible */
+					win.mode |= MODE_BLINK;
+				win.mode ^= MODE_BLINK;
+				tsetdirtattr(ATTR_BLINK);
+				lastblink = now;
+				timeout = blinktimeout;
+			}
+		}
+
 		draw();
 		XFlush(xw.dpy);
-
+		drawing = 0;
-			if (xev && !FD_ISSET(xfd, &rfd))
-				xev--;
-			if (!FD_ISSET(ttyfd, &rfd) && !FD_ISSET(xfd, &rfd)) {
-				if (blinkset) {
-					if (TIMEDIFF(now, lastblink) \
-							> blinktimeout) {
-						drawtimeout.tv_nsec = 1000;
-					} else {
-						drawtimeout.tv_nsec = (1E6 * \
-							(blinktimeout - \
-							TIMEDIFF(now,
-								lastblink)));
-					}
-					drawtimeout.tv_sec = \
-					    drawtimeout.tv_nsec / 1E9;
-					drawtimeout.tv_nsec %= (long)1E9;
-				} else {
-					tv = NULL;
-				}
-			}
-		}
 	}
 }