kids are doing something to change the way the actions editor works?

Hi team, 

Sometimes in my class (like today) someone does something in animate and the behaviour of the actions panel changes- instead of having a up and down cursor like I have here when I am typing the cursor becomes an underline and you cant put in any text, it only overwrites what is there - so if I wanted to put an underscore at the start of a word, like start, that was already in the code (to make start into _start) you would have to rewrite the whole line as when you put the _ in at the start it would delete the s.

By shutting down the whole program and restarting you can get back to the default but 

Q1.  What is this and how is it useful?

Q2 How do you turn it on and off?

Thanks and have a great day