Just left click and start highlighting till you hit the bottom of the visible part of the transcript window and drag it for a second or two past.
This will highlight the ENTIRE transcript all the way to the bottom.
Right click to copy [as you know].
I used to spend extra time dragging the highlighting all the way to the bottom of the transcript till I figured out by accident that once you started dragging the highlight, it had already gone to the bottom unseen.
No big deal on a short transcript - but an hour transcript was a real drag [pun intended].
Thanks, I get it.