compute between bytes and chars to get correct clipboard copy

line.py

@@ -4,18 +4,26 @@ class Line:
         self._byte_end = byte_end
         self._line = line
 
-    def byte_offset(self):
+    def byte_offset(self) -> int:
         return self._byte_offset
 
-    def byte_end(self):
+    def byte_end(self) -> int:
         return self._byte_end
 
-    def line(self):
+    def line(self) -> str:
         return self._line
 
-    def length(self):
+    def length(self) -> int:
         return len(self._line)
 
+    def char_index_to_byte(self, char_in_line: int) -> int:
+        return len(self.prefix(char_in_line).encode("utf8"))
+
+    def byte_index_to_char_index(self, byte_index: int) -> int:
+        prefix_bytes = self._line.encode("utf8")[:byte_index]
+        prefix_chars = prefix_bytes.decode("utf8", errors="ignore")
+        return len(prefix_chars)
+
     def includes_byte(self, byte: int) -> bool:
         return self._byte_offset <= byte <= self._byte_end