I'm also pondering a different pop-up situation. Instead of a big window with a message, something more like:
Enemy triggers AOO. Game pauses. Your Melee button appears on the UI flashing. First time this happens, tooltips tells you this means you can use your 1 reaction to trigger that ability. If you click the button, you use the reaction.
So, enemy is casting Fireball. Game pauses and UI jumps to spells and Counterspell highlights. Click it and use Counterspell. No message. No big interrupting sign. Just the flashing button. Don't want to use it? Hit spacebar or End Turn. So it's even faster than a pop-up. Literally, the UI just flashes and shows you exactly what you can do to react by highlighting the buttons.
We could always go for both. Pop-ups can be very helpful for people who have a difficult time understanding what's going on (or know they're going to step away from the computer for a bit and want to come back to a message explaining exactly what's going on), while your solution can be an option for those who already recognize what's happening at a glance.
Any way that this is presented works, as long as the core ability to
control how you react is there.
