This is not really a rules question, so I wasn't sure if this was the right place, but I thought it might fit better here.
Which interactions do we know of where we are in a "trigger window" of resolving when-x abilities, and this makes another when-x ability trigger?
• Classic example: Attack is played, react with Secret Chamber or Diplomat, draw another Reaction-to-Attack card.
Others:
• Kinda the same: Attack is played, react with Caravan Guard, draw another Reaction-to-Attack card.
• Trash a Cultist/Rats/Overgrown Estate, we are in when-trash window: Discard Market Square, draw cards, drawing another (or same) Market Square.
• Start-of-turn window: Play a card (via Ghost/Innovation/Piazza/Prince/Summon). If it's a Reserve card like Ratcatcher, can call it now. If it's Hireling, Prince, Fool gaining Lost in the Woods, or Treasurer gaining Key, the start-of-turn ability starts now.
• Gain a Duplicate, we are in when-gain window. Play it via Innovation, it can now be called to gain a copy of itself. (Or gain Vassal, play it, it plays Duplicate, call Duplicate to gain another Vassal.)
• Buy a card, we are in when-buy window: Gain another card (via Charm, Haggler etc) and play it via Innovation. If the played card is Goons, Haggler, Hoard*, Merchant Guild or Talisman*, it now triggers based on the buy.
*played via Black Market or Storyteller.
• Inherit Groundskeeper. Gain an Inherited Estate, we are in when-gain window. Play it via Innovation, it now triggers based on the gain. (Or Inherit Vassal, gain & play it, it plays Groundskeeper.)
Can anybody think of other interactions?