When I first made this extension, there was no way to hide all the activity details on cards. The list was full of entries like “So-and-so moved this from X list to Y list” and “So-and-so completed X item on this card”. Useful comments left by users on the card got lost.

Since that first release, Trello added a “Hide Details” button which hides activity by default on all cards. Which is great! The only trouble is that this choice is not remembered if you log out or clear the cookies on your browser. This extension activates the Hide Activity button, if it’s displayed, when you open a card.