Hello Tazz,
Now it works. First I tried to inherit class TSynGutteLineNumber without success, because overwrite of procedures does not work. As far as I can see, address of classes are saved into a list and there are launched from that list, so overwrite has no effect, strange method ! Afterward I included the source class in my application. That generates some problems of circular references with my Main Form and SynEdit class. Finally with declaring usage of main form unit into implementation paragraph of TSynGutterLine Number it works with changed TSynEditLineNumber Paint procedure. Thanks a lot for the leads you gave to me.