This is a very rare occurrence (only matters for Diadem), but came up on Discord. Champion's defense/+1 Action ability should resolve before the Attack/Action is played. This "first" ruling is per Donald X. here (http://forum.dominionstrategy.com/index.php?topic=18039.msg737985#msg737985). However, the client is currently executing the +1 Action ability after the Action is played.
This mattered when Champion was King's Courted (to power up Diadem). King's Courting a Champion should result in 6 Actions, but the client gives 3 additional Actions after you finish playing King's Court for 9 total. (Tip for investigating this: having a Coin of the Realm or Royal Carriage on your Tavern mat will let you see the number of Actions after each play of Champion, as the client pauses to ask if you'd like to call one of them.) It was a confusing discussion on Discord, so let me know if I'm not making sense recapping it here!
You make perfect sense AFA the topic allows. Stef is aware of it AFAIK, but it just is such a tiny minority of situations where it applies. Also, from recent discussions I think this body of rulings is not really stable, but I might be misremebering things.
Yes, definitely not a priority, just reporting it for completeness! And that ruling is indeed a few years old, so it could be worth checking with Donald X.