Where is the unerase function
 

As I am typing a response to a post, or deleting part of my post that I don't need, my computer has a habit of moving to another part of the post and unless I look up and notice it, I don't know it has jumped to another line. So if I go to "undo" it, I don't see a function to do that. The circle is to reload the page, which I don't want. Is there a symbol to click on to undo my screw-ups?

CTRL Z should, might undo

This works, verified. It generally works for any text field.

ok, so let me try this. I will try to type this in and then go back and erase some of it because my cursor moved.

hmmm, it does seem to work on a fake post