// does not end in the middle of a possibly-valid UTF-8 sequence.
//
// If a large text buffer must be split before position i at the latest,
// splitting at position trimUTF(b[:i]) avoids splitting a UTF-8 sequence.
func trimUTF8(b []byte) int {
	// Scan backward to find non-continuation byte.
	for i := 1; i < utf8.UTFMax && i <= len(b); i++ {
		if c := b[len(b)-i]; c&0xc0 != 0x80 {