Does spreading flames buff retroactively apply to links that have already resolved? The two main spots this would be relevant would be like:
Fai plays a blue brand with cinder claw, and uses ancestral empowerment to make it 2 power. It is blocked by a “That All You Got?”. The Fai continues the chain with the newly drawn spreading flames. When the combat chain closes, I don’t think the defending player would not draw a card (since the first attack is now 3 power).
The other situation would be like Spreading Flames, Spreading Flames, Blaze headlong. Blaze headlong is defended by phantasmal footsteps. If the Fai reaches 5 draconic links, I believe the footsteps would be destroyed when the combat chain closes (since the blaze headlong is now 6 power).
#Spreading Flames - Retroactive buffs
sterile sedge
desert trellis
Spreading flames + blaze headlong is 5 power not 6. But yes that effect is static so should apply retroactively (but will not change the damage already dealt)
charred valve
it might however pop phantasm
desert trellis
How would it pop phantasm when you are attacking?
charred valve
good point im an idiot 😄
and his example is double flames + headlong to blow up the footstep which would work
desert trellis
Ah missed the second in it
distant lodge
Spreading Flames' ability becomes functional when it resolves off the stack and is currently in the arena. Once it is in the arena, its effect to give Draconic Attacks you control +1 
will be applied to each attack that satisfies the conditions stated after "while" part of the clause (i.e. While their base is less than the number of Draconic chain links you control) [CR 1.8.3c] So it can retroactively affect attacks that are present on the combat chain.
Fai plays a blue brand with cinder claw, and uses ancestral empowerment to make it 2 power. It is blocked by a “That All You Got?”. The Fai continues the chain with the newly drawn spreading flames. When the combat chain closes, I don’t think the defending player would not draw a card (since the first attack is now 3 power).
In this scenario, you are correct that the defending player would not draw the card from the trigger because the static ability of "That All You Got?" is no longer functional because the Brand with Cinderclaw (blue) attack it is defending became 3
The other situation would be like Spreading Flames, Spreading Flames, Blaze headlong. Blaze headlong is defended by phantasmal footsteps. If the Fai reaches 5 draconic links, I believe the footsteps would be destroyed when the combat chain closes (since the blaze headlong is now 6 power).
Correct here too.