@romanrieme Sorry, I was rushing out the door earlier, and my fingers didn't type what was in my mind.
What I meant was that you could use [F2(t),F3(t)] in the `if`, instead of say just F2(t).
animate(display, [`if`(t < 0, F1(t), [F2(t), F3(t)]), scaling = constrained],
t = -1 .. 1, frames = 141)
note: In your followup comment you wrote, "F1(t) from -1 to 0", but in your worksheet your F2 appears in the subsection captioned, "t from -1 to 0". In any case I feel sure you can work out which you want where.
note: I am not so keen on the following, since two back-to-back frames are displayed each with t=0.
display([animate(F2, [t], t = -1 .. 0), animate([F1, F3], [t], t = 0 .. 1)], insequence)
note: My earlier suggestion display(F2(t),F3(t)) was a mental slip. It should be obvious why it would not work: t is just a name and F2(t) returns unevaluated which is useful elsewhere but not good for the display call. Read my correction here.