Is there anything at home that will provoke this reaction that you can use to help you install some prevention? Probably there is something that gets her attention such that she breaks her focus on you. Maybe not to the degree that she does at a show, but something that is distracting and that she is reluctant to take her eyes away from. It’s possible that she’s doing this at home but as she can multi-task, as it were, and basically keep you satisfied while in fact paying more attention to irrelevant distractions, you are not reacting to it.
Having spent years on one of the world’s spookiest horses, my conclusion was this: Fundamentally, the problem is Loosing Focus aka Distractability, followed up with Fixation, that is, a difficulty in returning focus to the rider/handler. All horses will always be aware of other things, it’s the way nature made them. But they have to be able to come back immediately to the rider/handler. THAT is the thing that has to be trained - yes, dear horse, you saw it, but this other thing is what we are doing now. (High energy, impulsive types of dogs, and people, have to work on the same thing.)
As a trainer once explained to me, practice always always always that whenever something else gets her attention, make sure she can break away and check back with you. Always. Relentlessly. While leading in from pasture, while grooming, while longeing, and of course while riding. You don’t get a break from reminding her about this because she never takes one. She keeps being a horse and this is an innate horse behavior that she may have more deeply ingrained than many other horses.
My thought is that this is like a teenager who can’t break away from the cell phone screen once they catch a glimpse. That is a learned habit. It is also a learned habit to be able to break away. Once the fixation-on-distraction habit is thoroughly ingrained, it is honestly very hard to install the ability to break away and return to the task at hand. It can be done. But is never completely done once the habit is solid, breaking away has to be re-schooled over and over again, forever.
I learned that a big key to a coming distraction while being ridden is that the horse’s head and eyes are not truly in the line of travel. For example, if bending to the right, the body and neck are bent to the right, except right at the poll where there is a little twitch and the horse’s eyes are looking outside the circle, not in the direction of travel. If going down the long side the head and eyes are slightly looking outside the ring. Approaching a jump the horse seems to abruptly forget where they are going as their attention is diverted to something else outside the course, and then is startled to find the jump is much closer than it was a second or two ago. All horses are inclined to do this as they are watchful reactive prey animals. If a rider doesn’t correct it and install focus, this will become a huge hole in their training. A rider can paddle around the easier courses and dressage tests with a funky distractable horse, but once you start increasing the level of difficulty, things begin to unravel because the horse is not truly focused on what they were doing. It is so much harder to fix this then than it is to fix it from the start. And every rider of this horse has to manage it consistently - everyone has to keep this horse on task and off of even minor distractions, because the horse is always escalating distractability.
My last thought is that I can’t emphasize how serious it is to her future career that a way is found to correct the problem in short order. Otherwise it will only endure and even get worse. As you are discovering, any horse that is so distractable as to be uncontrollably spooky, fixated on distractions and difficult to keep on track and forward, is virtually useless as a show horse in any discipline. Now it is just a quirk, but it will cost many failed weekends. The behavior is inclined to escalate without correction, and lead to enormous frustration after the rider has invested so much time and money into preparation, only to have something ridiculous cause the horse to spook and balk uncontrollably at a moment that flushes the entire weekend. Such a horse won’t be easy to re-home, although there are riders that are pretty good at managing this and maybe one will be willing to take her on. The horse’s long-term prospects are not good if they are not a pleasure to ride.
There are working dog breeds that are prone to this behavior. Some will fixate on sunspots and/or moving shadows in the middle of a task. They ignore the handler to watch whatever it is that grabs their attention. They don’t remember what they were doing before the distraction. This is a well-known tendency in some types of performance dogs. In some cases the habit is so bad that an expensive, professionally trained scent dog or other working dog is unable to ever perform the career that they have been carefully prepared to do. I think that the mental roots are at least somewhat the same as a spooky, distractable, fixated-on-everything-but-the-task horse. Some of the same training techniques were very useful with my spooky horse.
Good luck with this, hope you are able to resolve it and enjoy your lovely mare! 
