Difference between 溢れていた and 溢れている


瞬間ーーいつの間にやら開放感に **溢れていた** 教室の外から、折紙が現れる。

Hi. Is the bold ていた used here because the action 溢れる happened before the
action 折紙が現れる? And the state of being open-air continued until 折紙が現れる?

If so, can we use 溢れている here? Because, according to the context, the the wall
of the classroom has been destroyed so the classroom is still being open-air
at the time when 折紙 appears and the open-airness is going to continue.

And can I think, compared with 溢れている, 溢れていた is focusing more on the period
prior to the appearance of 折紙? And 溢れている is focusing more on the period at or
after the time when 折紙 appears? Thank you.

