auto-sync: draw on idle to avoid flicker/tearing

st could easily tear/flicker with animation or other unattended
output. This commit eliminates most of the tear/flicker.

Before this commit, the display timing had two "modes":

- Interactively, st was waiting fixed `1000/xfps` ms after forwarding
  the kb/mouse event to the application and before drawing.

- Unattended, and specifically with animations, the draw frequency was
  throttled to `actionfps`. Animation at a higher rate would throttle
  and likely tear, and at lower rates it was tearing big frames
  (specifically, when one `read` didn't get a full "frame").

The interactive behavior was decent, but it was impossible to get good
unattended-draw behavior even with carefully chosen configuration.

This commit changes the behavior such that it draws on idle instead of
using fixed latency/frequency. This means that it tries to draw only
when it's very likely that the application has completed its output
(or after some duration without idle), so it mostly succeeds to avoid
tear, flicker, and partial drawing.

The config values minlatency/maxlatency replace xfps/actionfps and
define the range which the algorithm is allowed to wait from the
initial draw-trigger until the actual draw. The range enables the
flexibility to choose when to draw - when least likely to flicker.

It also unifies the interactive and unattended behavior and config
values, which makes the code simpler as well - without sacrificing
latency during interactive use, because typically interactively idle
arrives very quickly, so the wait is typically minlatency.

While it only slighly improves interactive behavior, for animations
and other unattended-drawing it improves greatly, as it effectively
adapts to any [animation] output rate without tearing, throttling,
redundant drawing, or unnecessary delays (sounds impossible, but it
works).

config.def.h

@@ -43,9 +43,14 @@ 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;
-static unsigned int actionfps = 30;
+/*
+ * draw latency range in ms - from new content/keypress/etc until drawing.
+ * 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

x.c

@@ -1867,10 +1867,9 @@ 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 ttyfd;
-	struct timespec drawtimeout, *tv = NULL, now, last, lastblink;
-	long deltatime;
+	int xfd = XConnectionNumber(xw.dpy), ttyfd, xev, drawing;
+	struct timespec seltv, *tv, now, lastblink, trigger;
+	double timeout;
 
 	/* Waiting for window mapping */
 	do {
@@ -1891,51 +1890,31 @@ run(void)
 	ttyfd = ttynew(opt_line, shell, opt_io, opt_cmd);
 	cresize(w, h);
 
-	clock_gettime(CLOCK_MONOTONIC, &last);
-	lastblink = last;
-
-	for (xev = actionfps;;) {
+	for (timeout = -1, drawing = 0, lastblink = (struct timespec){0};;) {
 		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 (blinktimeout && TIMEDIFF(now, lastblink) > blinktimeout) {
-			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 (FD_ISSET(ttyfd, &rfd))
+			ttyread();
 
-		if (dodraw) {
+		xev = 0;
 		while (XPending(xw.dpy)) {
+			xev = 1;
 			XNextEvent(xw.dpy, &ev);
 			if (XFilterEvent(&ev, None))
 				continue;
@@ -1943,30 +1922,45 @@ run(void)
 				(handler[ev.type])(&ev);
 		}
 
-			draw();
-			XFlush(xw.dpy);
-
-			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;
-				}
+		/*
+		 * 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;
 	}
 }