Fix crashes during scrolling

- Occured due to negative scroll positions or not finding the
  equivalent line in the other editor.

BUGS

@@ -1,5 +1,4 @@
 Current:
-- Error sometimes when paging to the bottom of the file.
 
 
 Fixed:
@@ -8,6 +7,7 @@ Fixed:
 - When largest line grows the bounding box should grow.
 - Input can be lost when typing quickly and diff-edit is busy.
   Seen with rapid undo while diffing.
+- Error sometimes when paging to the bottom of the file.
 
 
 Won't fix:

diff_edit/__init__.py

@@ -182,15 +182,14 @@ class DiffEditor:
             if left_start <= y < left_end:
                 fraction = (y - left_start) / (left_end - left_start)
                 return round(right_start + fraction * (right_end - right_start))
+        return 0
 
     def follow_scroll(self):
         x, y = self.editors[0].scroll_position
         last_width, last_height = self.last_dimensions
         middle_y = last_height // 2
         new_y = self._equivalent_line(y + middle_y)
-        if new_y is None:
+        self.editors[1].scroll_position = max(0, x), max(0, new_y - middle_y)
-            new_y = 0
-        self.editors[1].scroll_position = max(0, x), new_y - middle_y
 
     def switch_editor(self):
         self.editors[1].cursor_x = self.editors[0].cursor_x

diff_edit/editor.py

@@ -658,7 +658,7 @@ class Editor:
 
     def scroll(self, dx, dy):
         view_x, view_y = self.scroll_position
-        self.scroll_position = view_x + dx, view_y + dy
+        self.scroll_position = max(0, view_x + dx), max(0, view_y + dy)
 
     def on_mouse_press(self, x, y):
         view_x, view_y = self.view_widget.position