Applying the CBT patch of Roberto Vargas. Thanks.

st.c

@@ -262,7 +262,7 @@ static void tinsertblankline(int);
 static void tmoveto(int, int);
 static void tnew(int, int);
 static void tnewline(int);
-static void tputtab(void);
+static void tputtab(bool);
 static void tputc(char*);
 static void treset(void);
 static int tresize(int, int);
@@ -1243,7 +1243,7 @@ csihandle(void) {
 	case 'I': /* CHT -- Cursor Forward Tabulation <n> tab stops */
 		DEFAULT(csiescseq.arg[0], 1);
 		while (csiescseq.arg[0]--)
-			tputtab();
+			tputtab(1);
 		break;
 	case 'J': /* ED -- Clear screen */
 		sel.bx = -1;
@@ -1356,7 +1356,11 @@ csihandle(void) {
 		DEFAULT(csiescseq.arg[0], 1);
 		tdeletechar(csiescseq.arg[0]);
 		break;
-	/* XXX: (CSI n Z) CBT -- Cursor Backward Tabulation <n> tab stops */
+	case 'Z': /* CBT -- Cursor Backward Tabulation <n> tab stops */
+		DEFAULT(csiescseq.arg[0], 1);
+		while (csiescseq.arg[0]--)
+			tputtab(0);
+		break;
 	case 'd': /* VPA -- Move to <row> */
 		DEFAULT(csiescseq.arg[0], 1);
 		tmoveto(term.c.x, csiescseq.arg[0]-1);
@@ -1528,11 +1532,20 @@ strreset(void) {
 }
 
 void
-tputtab(void) {
-	unsigned x;
+tputtab(bool forward) {
+	unsigned x = term.c.x;
 
-	for (x = term.c.x + 1; x < term.col && !term.tabs[x]; ++x)
+	if (forward) {
+		if (x == term.col)
+			return;
+		for (++x; x < term.col && !term.tabs[x]; ++x)
 			/* nothing */ ;
+	} else {
+		if (x == 0)
+			return;
+		for (--x; x > 0 && !term.tabs[x]; --x)
+			/* nothing */ ;
+	}
 	tmoveto(x, term.c.y);
 }
 
@@ -1650,7 +1663,7 @@ tputc(char *c) {
 			sel.bx = -1;
 		switch(ascii) {
 		case '\t':
-			tputtab();
+			tputtab(1);
 			break;
 		case '\b':
 			tmoveto(term.c.x-1, term.c.y);