The workspace. A session can hold eight browsers, and the interface showed one of them.
Now the focused browser fills the big pane at full rate and the others sit under it as small previews. Click one to send this session's commands to it. Open another browser, or close one, without asking the model to do it for you.
One pane is live and one slow loop refreshes the others in turn, and that is arithmetic rather than a preference: a frame costs about 22 ms on a pipe that actions share, so eight panes at full rate would want 2.3 seconds of pipe for every second that passes. Whether you have two browsers or eight, the previews cost the same.
browser_list answers JSON now - session, focus, limit, note and browsers with id, running, focused and each one's tab urls. A browser that is not running is one the session declared and has not needed yet; the next command aimed at it starts it as the same person.