Returns the width el has to itself in a row it shares with an element that comes first, such as the tabs that leave room for the presence avatars. The first element counts at its content's natural width, since that depends on the theme's fonts and the locale, and the row's gap is taken off too. It returns null until measured.

Passed to useRoomBeside by PresenceRow, from its own props. Without both selectors, the hook measures nothing and returns null.