Like Facebook's little chat bubble notification, or like iMessage, so that opponents (in private games) can see when their opponent is typing.
I didn't know they had that. Classy.
PSA! Github added "emoji reactions" (which was TOTALLY NOT stolen from slack) so you can now show your appreciations for good ideas without giving us notifications. Please use it!
By commenting, I also wanted to subscribe to the thread. Sorry for the inconvenience.
I don't know what is a chat bubble.

Indicates that someone else in the chat is typing just now.
This is exactly what I was coming here to say. We need a typing indicator so the person waits to play a new opponent until he can see what the other person says.
I like to offer advice or say good game but the person is already gone.
During a game though it could be distracting. Maybe only when it is your turn to play or when the game is over does your typing cause the typing indicator otherwise you could distract the opponent this way.
This exists now.
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PSA! Github added "emoji reactions" (which was TOTALLY NOT stolen from slack) so you can now show your appreciations for good ideas without giving us notifications. Please use it!