diff --git a/input-keys.c b/input-keys.c
index feb62f6d..ebf61333 100644
--- a/input-keys.c
+++ b/input-keys.c
@@ -606,19 +606,34 @@ input_key_get_mouse(struct screen *s, struct mouse_event *m, u_int x, u_int y,
 		len = xsnprintf(buf, sizeof buf, "\033[<%u;%u;%u%c",
 		    m->sgr_b, x + 1, y + 1, m->sgr_type);
 	} else if (s->mode & MODE_MOUSE_UTF8) {
-		if (m->b > 0x7ff - 32 || x > 0x7ff - 33 || y > 0x7ff - 33)
+		if (m->b > MOUSE_PARAM_UTF8_MAX - MOUSE_PARAM_BTN_OFF ||
+		    x > MOUSE_PARAM_UTF8_MAX - MOUSE_PARAM_POS_OFF ||
+		    y > MOUSE_PARAM_UTF8_MAX - MOUSE_PARAM_POS_OFF)
 			return (0);
 		len = xsnprintf(buf, sizeof buf, "\033[M");
-		len += input_key_split2(m->b + 32, &buf[len]);
-		len += input_key_split2(x + 33, &buf[len]);
-		len += input_key_split2(y + 33, &buf[len]);
+		len += input_key_split2(m->b + MOUSE_PARAM_BTN_OFF, &buf[len]);
+		len += input_key_split2(x + MOUSE_PARAM_POS_OFF, &buf[len]);
+		len += input_key_split2(y + MOUSE_PARAM_POS_OFF, &buf[len]);
 	} else {
-		if (m->b > 223)
+		if (m->b + MOUSE_PARAM_BTN_OFF > MOUSE_PARAM_MAX)
 			return (0);
+
 		len = xsnprintf(buf, sizeof buf, "\033[M");
-		buf[len++] = m->b + 32;
-		buf[len++] = x + 33;
-		buf[len++] = y + 33;
+		buf[len++] = m->b + MOUSE_PARAM_BTN_OFF;
+
+		/*
+		 * The incoming x and y may be out of the range which can be
+		 * supported by the "normal" mouse protocol. Clamp the
+		 * coordinates to the supported range.
+		 */
+		if (x + MOUSE_PARAM_POS_OFF > MOUSE_PARAM_MAX)
+			buf[len++] = MOUSE_PARAM_MAX;
+		else
+			buf[len++] = x + MOUSE_PARAM_POS_OFF;
+		if (y + MOUSE_PARAM_POS_OFF > MOUSE_PARAM_MAX)
+			buf[len++] = MOUSE_PARAM_MAX;
+		else
+			buf[len++] = y + MOUSE_PARAM_POS_OFF;
 	}
 
 	*rbuf = buf;
diff --git a/tmux.h b/tmux.h
index a893bdb2..53084b8b 100644
--- a/tmux.h
+++ b/tmux.h
@@ -581,6 +581,12 @@ enum tty_code_code {
 #define MOTION_MOUSE_MODES (MODE_MOUSE_BUTTON|MODE_MOUSE_ALL)
 #define CURSOR_MODES (MODE_CURSOR|MODE_CURSOR_BLINKING|MODE_CURSOR_VERY_VISIBLE)
 
+/* Mouse protocol constants. */
+#define MOUSE_PARAM_MAX 0xff
+#define MOUSE_PARAM_UTF8_MAX 0x7ff
+#define MOUSE_PARAM_BTN_OFF 0x20
+#define MOUSE_PARAM_POS_OFF 0x21
+
 /* A single UTF-8 character. */
 typedef u_int utf8_char;
 
diff --git a/tty-keys.c b/tty-keys.c
index 8538e74b..64dd91bb 100644
--- a/tty-keys.c
+++ b/tty-keys.c
@@ -1061,17 +1061,13 @@ tty_keys_mouse(struct tty *tty, const char *buf, size_t len, size_t *size,
 		log_debug("%s: mouse input: %.*s", c->name, (int)*size, buf);
 
 		/* Check and return the mouse input. */
-		if (b < 32)
+		if (b < MOUSE_PARAM_BTN_OFF ||
+		    x < MOUSE_PARAM_POS_OFF ||
+		    y < MOUSE_PARAM_POS_OFF)
 			return (-1);
-		b -= 32;
-		if (x >= 33)
-			x -= 33;
-		else
-			x = 256 - x;
-		if (y >= 33)
-			y -= 33;
-		else
-			y = 256 - y;
+		b -= MOUSE_PARAM_BTN_OFF;
+		x -= MOUSE_PARAM_POS_OFF;
+		y -= MOUSE_PARAM_POS_OFF;
 	} else if (buf[2] == '<') {
 		/* Read the three inputs. */
 		*size = 3;