Sometimes I have to click "Done reacting" several times after playing an action, up to three times even with only one Carriage on my mat. This is quite annoying. Is there some situation in which this is useful, is this an unavoidable (or difficult to avoid) consequence of how it is programmed to behave in other situations, is it simply a bug?
Please somebody at least explain to me why it works the way it does.
It's not a bug, but it would often be avoidable. The reason is that the client strictly follows the card text and rules "after you finish playing an action card".
For example when you play Throne Room - Throne Room - Action Card, it must give you the choice to call it on the Action Card or on the Throne Room. But it wouldn't have to give you the choice on which Throne Room you call it (maybe there's some edge case, but usually at least).
Btw, Coin of the Realm is more annoying in that sense, because it's less often relevant after which action card in a stack you call it. The simplest way to improve this that comes to my mind would let the client automatically continue until new information would be revealed. Then, you could choose whether and on which action card you want to call it (since the last time new info has been revealed).
I'm not talking about Throne Room interactions, sometimes I just play a Rabble and have one Royal Carriage on my mat and then I have to click "Done reacting" three times to get to my buy phase.
If you have a game and turn number, it's easy to check. I don't recall experiencing such a situation.
I don't have a game #, but having to decline Royal Carriage twice is a common occurrence and having to decline it three times for the same action has happened to me before as well, surely Royal Carriage works this way for everyone...
I agree that it's easy to trigger it for Throne Room variants (incl. Royal Carriage itself), but I don't think it's a common occurence otherwise.
With Rabble (or other action cards) you can trigger this by calling multiple RC immediately after playing:
1) Play Rabble with at least 2 RC on tavern mat
2) You're done resolving Rabble and call Royal Carriage
3)a) You're done resolving the replayed Rabble and have to decline
3)b) You're still done resolving the original Rabble and have to decline (e.g. if it was your last action card in hand to enter buy phase)