Trim trailing whitespaces after processing ANSI sequences

Close #4282

src/util/chars.go

@@ -184,6 +184,11 @@ func (chars *Chars) TrailingWhitespaces() int {
 	return whitespaces
 }
 
+func (chars *Chars) TrimTrailingWhitespaces() {
+	whitespaces := chars.TrailingWhitespaces()
+	chars.slice = chars.slice[0 : len(chars.slice)-whitespaces]
+}
+
 func (chars *Chars) TrimSuffix(runes []rune) {
 	lastIdx := len(chars.slice)
 	firstIdx := lastIdx - len(runes)