fix computation of text width

line.py

@@ -22,4 +22,13 @@ class Line:
     def intersects(self, start_byte: int, end_byte: int):
         result = start_byte < self._byte_end and end_byte > self._byte_offset
         #print("%d,%d in %d,%d" % (start_byte, end_byte, self._byte_offset, self._byte_end))
-        return result
+        return result
+
+    def prefix(self, index: int) -> str:
+        return self._line[0:index]
+
+    def substr(self, offset: int, length: int) -> str:
+        return self._line[offset:offset+length]
+
+    def suffix(self, index: int) -> str:
+        return self._line[index:]