Unfortunately, blur, that's not quite related to the problem at hand. (Regarding the article, I would probably update it if I was more interested in doing so.)

If you are playing in Single Player, nothing that is more than 31 tiles away from the player will be kept track of. Enemies and bosses will regain full health, objects that came out of crates will disappear, and trigger scenery will fail to activate. This last one is usually no problem as it does activate once you come close enough to the tiles, and this happens before you even see the tiles, so on average you will never know there is a delay. The background trigger thing, however, displays the effects of trigger scenery in locations other than the actual tile they are placed in. Thus, to trigger background layers in Single Player, you must be within 31 tiles of the actual trigger scenery events in order to see the effects of the trigger.