In D&D 5e, A troll's regenerate ability is as follows (MM 291):
Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
Suppose a troll is brought down to 0 HP via a non-fire, non-acid damage source, but there are still several more turns before the start of the troll's next turn that would trigger its regeneration ability. During this time period, is the troll still standing and otherwise appearing to be alive, or does it fall down and appear to be dead the instant it hits 0 HP, but then use half of its movement to get up from being prone at the start of its next turn when its regeneration ability triggers?
Is there any official ruling on this, or is it the DM's preference?