Does Decree of Focus mistargets if the red unit in combat exits it? I attack Star Spring with one unit (Red) my opponent casts Decree of Focus on his unit, then I react with Nidalee and Star Spring triggers, and I send my red unit to base. How does this chain resolves?

Yes, Decree of Focus mistargets and gives no +4 Might.

Here's the chain resolution:

1. Bottom of chain: Decree of Focus (defender), targeting their unit at Star Spring. Target is legal — their unit is in combat with your red Fury unit.
2. Top of chain: Star Spring's trigger (from Nidalee entering via Ambush). It resolves first — you move your red unit to base.
3. A Cleanup occurs after that move. Your red unit is no longer at the combat battlefield, so it loses its Attacker designation (FAQ 2164; the unit that left is no longer "in combat").
4. Decree of Focus resolves: Its targeting requirement is "a friendly unit that's in combat with an enemy Fury unit." At this point, there is no enemy Fury unit in combat with the defender's unit — your red Fury unit is at base and out of combat. The target no longer meets the spell's targeting restrictions, so it's illegal, and the +4 Might instruction is ignored (Rule 359.3.e.2, 359.3.e.5).

Nidalee remains at Star Spring and will gain Attacker designation during the next Cleanup (Rule 464.2.c.3.a), but she doesn't save Decree of Focus — she's not a Fury unit, and the spell was already on the chain.