Wait only when the parsed sequence is the only one in the buffer

The two checks could refer to different sequences. In \e[3;\a\e[ the first
looked unfinished and so did the trailing \e[, though the BEL had already
ended the first. The blocking read held back the CTRL-G abort.

Reported by Copilot on #4926.

src/tui/light.go

@@ -444,12 +444,12 @@ func stringReply(buffer []byte) bool {
 	return false
 }
 
-// stillArriving reports whether the sequence that consumed sz bytes of the
-// buffer may still be completed by more input. Both it and the buffer have to
-// end unfinished: a complete sequence followed by ALT-[ is not waiting for
-// anything, and an unfinished one followed by other bytes cannot complete.
-func stillArriving(buffer []byte, sz int) bool {
-	return incompleteEscape(buffer[:min(sz, len(buffer))]) && incompleteEscape(buffer)
+// stillArriving reports whether the sequence at the start of the buffer may
+// still be completed by more input. It must be the only one in the buffer:
+// whatever follows it, another sequence or a byte that ended it, means it is not
+// waiting for anything.
+func stillArriving(buffer []byte) bool {
+	return bytes.IndexByte(buffer[1:], Esc.Byte()) < 0 && incompleteEscape(buffer)
 }
 
 // incompleteEscape reports whether the buffer ends in an escape sequence that
@@ -589,7 +589,7 @@ func (r *LightRenderer) GetChar(cancellable bool) Event {
 		// Second chance, but only for an unfinished sequence. Re-reading
 		// otherwise blocks until the next keystroke, holding back whatever
 		// follows in the buffer.
-		if ev.Type == Invalid && stillArriving(r.buffer, sz) {
+		if ev.Type == Invalid && stillArriving(r.buffer) {
 			r.buffer, result, err = r.getBytes(true)
 			if err != nil {
 				return Event{Fatal, 0, nil}

src/tui/light_csi_test.go

@@ -139,19 +139,19 @@ func TestStringEnd(t *testing.T) {
 func TestStillArriving(t *testing.T) {
 	for _, c := range []struct {
 		buffer string
-		sz     int
 		want   bool
 	}{
-		{"\x1b[97;5u\a\x1b[", 7, false},
-		{"\x1b[97;5u\x1b[", 7, false},
-		{"\x1b[1;\a", 4, false}, // the BEL has already ended it
-		{"\x1b[1;", 4, true},
-		{"\x1b[", 2, true},
-		{"\x1bO1", 3, true},
-		{"\x1b[<0;1", 3, true},
+		{"\x1b[97;5u\a\x1b[", false},
+		{"\x1b[97;5u\x1b[", false},
+		{"\x1b[3;\a\x1b[", false}, // the BEL ended it, the trailing \e[ is another one
+		{"\x1b[1;\a", false},      // the BEL has already ended it
+		{"\x1b[1;", true},
+		{"\x1b[", true},
+		{"\x1bO1", true},
+		{"\x1b[<0;1", true},
 	} {
-		if got := stillArriving([]byte(c.buffer), c.sz); got != c.want {
-			t.Errorf("stillArriving(%q, %d) = %v, want %v", c.buffer, c.sz, got, c.want)
+		if got := stillArriving([]byte(c.buffer)); got != c.want {
+			t.Errorf("stillArriving(%q) = %v, want %v", c.buffer, got, c.want)
 		}
 	}
 }