Word for that certain fear, worrying you might involuntarily jump from a cliff?

Those are called "intrusive thoughts" and are typical, but not exclusive, of Obsessive-Compulsive Disorder (OCD).

• Intrusive thoughts, in the spectrum of OCD, are where a person generally suffers with obsessional thoughts that are repetitive, disturbing and often horrific and repugnant in nature. For example, thoughts of causing violent or sexual harm to loved ones. Because the intrusive thoughts are repetitive and not voluntarily produced, they cause the sufferer extreme distress - the very idea that they are capable of having such thoughts in the first place can be horrifying. However, what we do know is that people with Obsessive-Compulsive Disorder are the least likely people to actually act on the thoughts, partly because they find them so repugnant and go to great lengths to avoid them and prevent them from happening. Intrusive thoughts can cover absolutely any subject: sexual, religious, relationships, violence, etc. OCD

Edit: As I said before, intrusive thoughts are not exclusive of OCD. Many people do have them occasionally. The difference is that in OCD they are repetitive.

Examples of common intrusive thoughts (from Wikipedia)

• causing harm to elderly people
• imagining or wishing harm upon someone close to oneself
• impulses to violently attack, hit, harm or kill a person, small child, or animal
• impulses to shout at or abuse someone, or attack and violently punish someone, or say something rude, inappropriate, nasty, or violent to someone.

For a complete text on the subject, Wikipedia

I've known this described as The Imp of the Perverse rather than being a phobia. The term comes from the short story of the same name by Edgar Allan Poe.

The Imp of the Perverse is a metaphor for the urge to do exactly the wrong thing in a given situation for the sole reason that it is possible for wrong to be done

I've had similar myself. Sometimes I think about driving my car into a tree when I'm rolling along. It's like thinking about defacing the Mona Lisa when you're right in front of it. If you're holding a friend's phone, you might think about throwing it off a bridge...

It's not that you're likely to do these things but it's certainly a very bizarre feeling.

I don't think that there is a medical or layman term for that kind of situation. However, I must say that that example is under the category of specific phobia- an irrational fear of an object or a situation.

But according from the example you gave, there is a French word that best describes the eerie feeling to jump.

L'appel du vide well known as "the call of the void" specifically refers to that feeling you get when you're at the top of a tall cliff and consider jumping off. It can be considered a form of self destructive ideation. It's actually fairly anxiety provoking for most people to be in that kind of a situation, one in which you are quite literally a step away from death.

Medically speaking, it is not a phobia because it does not fall in numerous types of phobia. However, according to a book entitled Videbeck Psychiatric Mental health Nursing 2nd edition it states that,

"A phobia is an illogical, intense, persistent fear of a specific object or a social situation that causes extreme distress and interferes with normal functioning. People with phobias understand that their fear is unusual and irrational..."

Therefore, it may fall as another example of specific phobia. Furthermore, l'appel du vide is actually an untranslatable word so it has no equal English translation.

I might as well add that the given examples above resembles of a phobia and not of a symptom of Obsessive-Compulsive Disorder.

"Obsessions are recurrent, persistent, intrusive, and unwanted impulses that cause marked anxiety. Compulsions are ritualistic or repetitive behaviors or mental acts that a person carries out continuously in an attempt to neutralize anxiety." Videbeck (2nd edition)

The definition itself tells that a person will be diagnosed with OCD if there are persistent and unwanted thoughts. And to neutralize the anxiety there must be a repetitive action or behavior like washing of hands every minute to prevent any microbial contamination. The situation given by the user who posted this question has no repetitive behavior. It just only shows that there is an irrational fear or unexplained urge to jump in high places.

Book source: Videbeck, S. (2003). Psychiatric mental health nursing (2nd ed.). Lippincott Williams and Wilkins: PA.