support tmux (sixel enabled) (#29)

* support tmux (sixel enabled)

* add comment

Author: mattn

term/sixel.go

@@ -25,10 +25,9 @@ func checkSixel() bool {
 		return true
 	}
 	s, err := terminal.MakeRaw(1)
-	if err != nil {
-		return false
+	if err == nil {
+		defer terminal.Restore(1, s)
 	}
-	defer terminal.Restore(1, s)
 	_, err = os.Stdout.Write([]byte("\x1b[c"))
 	if err != nil {
 		return false
@@ -40,6 +39,13 @@ func checkSixel() bool {
 	if err != nil {
 		return false
 	}
+	// Check tmux (sixel enabled)
+	// See: CSI Ps c, Ps = 4 (Sixel graphics)
+	// https://invisible-island.net/xterm/ctlseqs/ctlseqs.html
+	// NOTE: tmux does not return VT name.
+	if bytes.HasPrefix(b[:n], []byte("\x1b[?1;2;4c")) {
+		return true
+	}
 	var supportedTerminals = []string{
 		"\x1b[?62;", // VT240
 		"\x1b[?63;", // wsltty