Fix replace logic

Author: Jonathan Plasse

src/textual/widgets/_masked_input.py

@@ -623,7 +623,9 @@ def replace(self, text: str, start: int, end: int):
         self.cursor_position = start
         new_value = self._template.insert_text_at_cursor(text)
         if new_value is not None:
-            self.value, self.cursor_position = new_value
+            value, cursor_position = new_value
+            self.value = value[:cursor_position] + value[end:]
+            self.cursor_position = cursor_position
         else:
             self.cursor_position = old_cursor_position
             self.restricted()