Fix mousereport

This patch replaces the previous one I sent.

The following changes are made in this patch:
 - Fix tracking of pressed buttons. Previously, pressing two buttons and
   then releasing one would make st think no buttons are pressed, which
   in particular broke MODE_MOUSEMOTION.
 - Always send the lowest-numbered pressed button on motion events; when
   no button is pressed for a motion event in MODE_MOUSEMANY, then send
   a release. This matches the behaviour of xterm. (Previously, st sent
   the most recently pressed button in the motion report.)
 - Remove UB (?) access to potentially inactive struct member
   e->xbutton.button of XEvent union.
 - Fix (unlikely) possibility of overflow for large button numbers.

The one discrepancy I found between st and xterm is that xterm sometimes
encodes buttons with large numbers (>5) strangely. E.g., xterm reports
presses of buttons 8 and 9 as releases, whereas st properly (?) encodes
them as presses.

(cherry picked from commit ea7cd7b62fdfa6a1fbd882d1565d557577f2cf32)

x.c

@@ -249,7 +249,7 @@ static char *opt_line  = NULL;
 static char *opt_name  = NULL;
 static char *opt_title = NULL;
 
-static int oldbutton = 3; /* button event on startup: 3 = release */
+static uint buttons; /* bit field of pressed buttons */
 
 void
 clipcopy(const Arg *dummy)
@@ -361,61 +361,68 @@ mousesel(XEvent *e, int done)
 void
 mousereport(XEvent *e)
 {
-	int len, x = evcol(e), y = evrow(e),
-	    button = e->xbutton.button, state = e->xbutton.state;
+	int len, btn, code;
+	int x = evcol(e), y = evrow(e);
+	int state = e->xbutton.state;
 	char buf[40];
 	static int ox, oy;
 
-	/* from urxvt */
-	if (e->xbutton.type == MotionNotify) {
+	if (e->type == MotionNotify) {
 		if (x == ox && y == oy)
 			return;
 		if (!IS_SET(MODE_MOUSEMOTION) && !IS_SET(MODE_MOUSEMANY))
 			return;
-		/* MOUSE_MOTION: no reporting if no button is pressed */
-		if (IS_SET(MODE_MOUSEMOTION) && oldbutton == 3)
+		/* MODE_MOUSEMOTION: no reporting if no button is pressed */
+		if (IS_SET(MODE_MOUSEMOTION) && buttons == 0)
 			return;
-
-		button = oldbutton + 32;
-		ox = x;
-		oy = y;
+		/* Set btn to lowest-numbered pressed button, or 12 if no
+		 * buttons are pressed. */
+		for (btn = 1; btn <= 11 && !(buttons & (1<<(btn-1))); btn++)
+			;
+		code = 32;
 	} else {
-		if (!IS_SET(MODE_MOUSESGR) && e->xbutton.type == ButtonRelease) {
-			button = 3;
-		} else {
-			button -= Button1;
-			if (button >= 7)
-				button += 128 - 7;
-			else if (button >= 3)
-				button += 64 - 3;
-		}
-		if (e->xbutton.type == ButtonPress) {
-			oldbutton = button;
-			ox = x;
-			oy = y;
-		} else if (e->xbutton.type == ButtonRelease) {
-			oldbutton = 3;
+		btn = e->xbutton.button;
+		/* Only buttons 1 through 11 can be encoded */
+		if (btn < 1 || btn > 11)
+			return;
+		if (e->type == ButtonRelease) {
 			/* MODE_MOUSEX10: no button release reporting */
 			if (IS_SET(MODE_MOUSEX10))
 				return;
-			if (button == 64 || button == 65)
+			/* Don't send release events for the scroll wheel */
+			if (btn == 4 || btn == 5)
 				return;
 		}
+		code = 0;
 	}
 
+	ox = x;
+	oy = y;
+
+	/* Encode btn into code. If no button is pressed for a motion event in
+	 * MODE_MOUSEMANY, then encode it as a release. */
+	if ((!IS_SET(MODE_MOUSESGR) && e->type == ButtonRelease) || btn == 12)
+		code += 3;
+	else if (btn >= 8)
+		code += 128 + btn - 8;
+	else if (btn >= 4)
+		code += 64 + btn - 4;
+	else
+		code += btn - 1;
+
 	if (!IS_SET(MODE_MOUSEX10)) {
-		button += ((state & ShiftMask  ) ? 4  : 0)
-			+ ((state & Mod4Mask   ) ? 8  : 0)
-			+ ((state & ControlMask) ? 16 : 0);
+		code += ((state & ShiftMask  ) ?  4 : 0)
+		      + ((state & Mod4Mask   ) ?  8 : 0)
+		      + ((state & ControlMask) ? 16 : 0);
 	}
 
 	if (IS_SET(MODE_MOUSESGR)) {
 		len = snprintf(buf, sizeof(buf), "\033[<%d;%d;%d%c",
-				button, x+1, y+1,
-				e->xbutton.type == ButtonRelease ? 'm' : 'M');
+				code, x+1, y+1,
+				e->type == ButtonRelease ? 'm' : 'M');
 	} else if (x < 223 && y < 223) {
 		len = snprintf(buf, sizeof(buf), "\033[M%c%c%c",
-				32+button, 32+x+1, 32+y+1);
+				32+code, 32+x+1, 32+y+1);
 	} else {
 		return;
 	}
@@ -458,9 +465,13 @@ mouseaction(XEvent *e, uint release)
 void
 bpress(XEvent *e)
 {
+	int btn = e->xbutton.button;
 	struct timespec now;
 	int snap;
 
+	if (1 <= btn && btn <= 11)
+		buttons |= 1 << (btn-1);
+
 	if (IS_SET(MODE_MOUSE) && !(e->xbutton.state & forcemousemod)) {
 		mousereport(e);
 		return;
@@ -469,7 +480,7 @@ bpress(XEvent *e)
 	if (mouseaction(e, 0))
 		return;
 
-	if (e->xbutton.button == Button1) {
+	if (btn == Button1) {
 		/*
 		 * If the user clicks below predefined timeouts specific
 		 * snapping behaviour is exposed.
@@ -677,6 +688,11 @@ xsetsel(char *str)
 void
 brelease(XEvent *e)
 {
+	int btn = e->xbutton.button;
+
+	if (1 <= btn && btn <= 11)
+		buttons &= ~(1 << (btn-1));
+
 	if (IS_SET(MODE_MOUSE) && !(e->xbutton.state & forcemousemod)) {
 		mousereport(e);
 		return;
@@ -684,7 +700,7 @@ brelease(XEvent *e)
 
 	if (mouseaction(e, 1))
 		return;
-	if (e->xbutton.button == Button1)
+	if (btn == Button1)
 		mousesel(e, 1);
 }