This is how chat looks for me on a tablet:
The major problem is that the sidebar is not shown completely, which makes the list of my other rooms and the starred messages inaccessible. Not seeing the list of starred messages is not so bad, but the list of my other rooms is very useful. It allows me to quickly see if anything is happening in one of the other rooms and makes switching between rooms easy.
The mobile interface feels rather restrictive on a tablet and also wastes a lot of space in landscape orientation.
One solution would be to make the sidebar elements collapsible and allow users to hide the room description, the list of present users and all other elements. Or if not enough vertical space is available, some elements could be made smaller automatically. The list of users could maybe only show one row if there isn't much space, instead of the four rows I'm seeing now sometimes.