What is the "A" button on the PC?

What is the "A" button on the PC?

I'm trying to finish the deckers.die mission on the PC. When I get to the
boss fight, it tells me to mash the "A" button. I hit the "A" key, and
that doesn't work. The few times it tells me to hit "Space" that does
work. So, I know my timing is good.
I looked through the Control bindings, and I don't see it. I do see in the
gamepad section, that the default "A" button action is to jump, which is
the same as "Space" on the PC. Is that what I should be hitting?
So, my question is, What is the "A" button on the PC?
----- edit
When I hit Esc to get to the menu, one of the choices is highlighted with
a green "A" beside it, like in the fight. Hitting "Space" does nothing.
So, that's not it.
----- 2nd edit
As my answer below says, "Space" did work during the actual in-game fight