On the second read I am wondering
Problem: The left arrow key have troubles (running on El Capitan).
I wanted to lauch XCode, so I booted on High Sierra and after sometimes, I continued to make some work in a large folder and between others, I make massive renames that I cannot automatize.
After one hour or so, I noticed that the Left Arrow Key apparently works fine.
Conclusion #1: after more teesting, I am now sure I do not have a hardware failure on the use of that key.
Conclusion #2: until I found a bad software, I probably have some application that bugs my Left Arrow Key.
Now, this may be a virus, a bacteria, a troyan, whatever name is not important (for me): I only want to remove what bugs me.
And as I wrote above, there are rooms to do these.
Set Channel: Off Topic.