It may be easier to just describe what you think is different and let us confirm/disprove...
Well, for years I do only use reaction fire and grenades in all 3 X-Com games (I even fixed Xcom Apoc in ASM to get rid of a bug cutting down the TUs), but when I now tried to play this in open xcom, I saw there must be kinda "bonus" if the actor has LOS to the spotter.
In xcom 1, it someties seemed my guys did fire on spotted enemies even before they took any action.
They probably turned or did something else other than fireing/moving to trigger this.
In open xcom/extended, however, I often see enemy units like reapers to step out of a door with, probably, the last few TUs, as it stops then. And no one of my elite is making a shot although all have full TUs left.
Countless other occasions show enemies step out of UFO and shoot (or move one more tile) before my guys get to react.
If, however, my guys are aside the door where the actor has no LOS to them, my guys fire away like hell directly on sighting.
So I conclude the very first instance of reaction is denied if the actor has LOS to the spotter.
This is quite new behaviour to me.
I did not go into detail before because I am probably the only one playing this way, so this would be of no interest to anyone else.