Bug: Unified Highlighter does not care about fragment_size

[aslamy]

I have:

Elasticsearch 6.1.2

Mapping:

PUT my_index
{  
   "mappings":{  
      "my_type":{  
         "properties":{  
            "title":{  
               "term_vector":"with_positions_offsets",
               "type":"text"
            }
         }
      }
   }
}

PUT my_index/my_type/1
{  
   "title":"Hello. This is a test."
}

Search:

{  
   "query":{  
      "match":{  
         "title":"Hello"
      }
   },
   "highlight":{  

      "fields":{  
         "title":{  
            "fragment_size":1000,
            "number_of_fragments":1,
	    "type": "unified"
         }
      }
   }
}

Result:

{  
   "took":0,
   "timed_out":false,
   "_shards":{  
      "total":5,
      "successful":5,
      "skipped":0,
      "failed":0
   },
   "hits":{  
      "total":1,
      "max_score":0.2876821,
      "hits":[  
         {  
            "_index":"my_index",
            "_type":"my_type",
            "_id":"1",
            "_score":0.2876821,
            "_source":{  
               "title":"Hello. This is a test."
            },
            "highlight":{  
               "title":[  
                  "<em>Hello</em>."
               ]
            }
         }
      ]
   }
}

Problem:
It seems that unified highlighter does not care about fragment size. When fragment_size is 1000 it should not split but it does split on first point (dot). I know if fragment_size = 0 then it not split, but that's not what I want.
I have fields with many sentence I want to split around 256 characters, but highlighter splitting always on first point hit.

[jimczi]

This is expected. In this version the unified highlighter splits the text on sentence boundaries and creates one fragment per sentence or multiple fragments if the sentence is bigger than fragment_size.
Though we changed the behavior in 6.2 where it will be able to select all sentences that fit into fragment_size:
#28132