I've noticed that occasionally when the first tab in the script editor opens up, the scrollbar may only allow you to scroll down to a certain point (I estimate around 30-40 lines) and you have to open a second tab, close the first, (Optionally close the second) reopen the one you originally wanted to edit before the scrollbar will allow you to go right to the end of the script.
Edit: Will post a screenshot when I can replicate this, but it is really easy to replicate.
Edit 2:
Using CTRL + End, you are able to make the cursor go to the bottom, though you cannot see it. The bottom visible line is line 59 and doesn't allow you to scroll any lower unless you do the above mentioned actions.