code: function showDiff(acediff, editor, startLine, endLine, className) {
    var editor = acediff.editors[editor];
  
    if (endLine < startLine) { // can this occur? Just in case.
      endLine = startLine;
    }
  
    const classNames = `${className} ${(endLine > startLine) ? 'lines' : 'targetOnly'}`;
    endLine--; // because endLine is always + 1
  
    // to get Ace to highlight the full row we just set the start and end chars to 0 and 1
    editor.markers.push(editor.ace.session.addMarker(new Range(startLine, 0, endLine, 1), classNames, 'fullLine'));
  }
docs: shows a diff in one of the two editors.