When an ignored user joins a chat room, their avatar is shown in half size in the user list to the top right. The container that contains the avatar retains its full height, so everything is pushed down by half an avatar's height on my screen when an ignored user joins.
More specifically, the ignored class seems to be added on the user container. It resizes the avatar, but the height of the container is governed by #present-users > .present-user, #present-users .more, which makes the height 32 pixels. I believe .present-user.ignored should set the height to 16px to match .ignored .user-gravatar32.
