When a user open the channels list dialog, he wants to look at the channels on the server. There is no reason to force him to hit the refresh button to load them. So the channels list should be automatically loaded when dialog is opened.

If he finds a interesting channel, there is good chance than he wants to join it. In fact, search and join channels are the main purposes of this dialog, so the "join channel" button shouldn't be so far from the channels list.

It would be also fine than double click on a channel join it.