Fix crash when switching to non-existant line

- When switching from an insert area at the bottom of a file.

diff_edit/__init__.py

@@ -209,7 +209,8 @@ class DiffEditor:
                     right_start, right_end, left_start, left_end
             if left_start <= y < left_end:
                 fraction = (y - left_start) / (left_end - left_start)
-                return round(right_start + fraction * (right_end - right_start))
+                other_y = round(right_start + fraction * (right_end - right_start))
+                return other_y - 1 if other_y == len(self.editors[1].text_widget) else other_y
         return 0
 
     def follow_scroll(self):

diff_edit/editor.py

@@ -263,6 +263,7 @@ class Editor:
         if not self.is_editing:
             return result
         if self.mark is None:
+            with contextlib.suppress(IndexError):  # Fix. The cursor should always be on screen.
                 result[self.cursor_y - view_y] = highlight_str(result[self.cursor_y - view_y],
                                                                termstr.Color.white, 0.8)
         else: