The diff engine allows you to configure how much context you want to display using the diff_context_lines_leading and diff_context_lines_trailing variables, but the variable system will not be available in Drupal 8.
Convert the following lines to use CMI:
1099 $this->leading_context_lines = variable_get('diff_context_lines_leading', 2);
1100 $this->trailing_context_lines = variable_get('diff_context_lines_trailing', 2);
There is no UI to set these variables.
User interface changes
This is a child of