Your league in the sidebar
Open a league and it stays open. Its divisions, seasons, tournaments, draws, members, Discord settings and league settings move into the sidebar and stay there while you work, including on the dashboard and your fixtures.
What changed
The league page has lost its row of buttons. There were nine of them across the top, and every page they led to needed a Back to league button to get you home again. Both are gone. What the page carries now is what you can do about a league — join it, or leave it.
The sidebar knows which league you are in. Opening one from the picker, from a link somebody pasted, or from a notification puts its menu below the main navigation. It stays until you open a different one.
Only leagues you belong to are remembered. Reading somebody else's league shows you their menu while you are on their pages, and does not move you out of your own.
Members is the new name for what the sidebar calls the page that was headed Manage members. Nothing on it has changed.
The Leagues page
The page was doing two jobs. It listed the leagues you run and play in beside every league you could join, in one table, with a column reading either your role or Not a member.
The leagues you are in are now cards. Each one shows the league's logo, how many members it has, how many divisions, and which season is under way — the things you want before deciding where to go next. A card for starting a new league sits at the end.
The leagues you could join are still a table, under their own heading, with the same Join button. A league set to By invitation is still listed nowhere.
Elsewhere
The dashboard no longer lists your leagues at the bottom. The picker does that, and the sidebar carries whichever one you are working in, so the third copy was one too many. An account that is in no league still gets the way into one.