To me, it's a matter of timing. If the first frames, words, etc. were just coming on the screen as you walked in, I'd say "was starting" is correct/accurate. However, if content had been on the screen for 20 or 30 seconds, then I believe "had started" would be more accurate.

Whether you use "when" or "by the time" is to me, a matter of preference. I've used both, but I believe "when" would be more "acceptable" to a greater number of critics.

Change of subject: the use of "what makes" rather than "that makes" in the last sentence is for me, a faux pas.

WOW ! :) I thoroughly enjoyed the bring/take dialogue and was pleased to see that I am not alone in my annoyance. I actually learned quite a lot/bit as well, so a hearty THANKS to everyone (one-and-all :).