# Community sidebar

When a community is opened from the sidebar, the sidebar's content becomes the
community's own channel/member browser. Going back restores the normal tabs.

```
+------------------+--------------------------------+
| [<-] community   |                                |
| banner           |                                |
| v Channels  3    |   messages                     |
|   # general      |                                |
|   # random       |                                |
| v Admins    1    |                                |
|   @owner         |                                |
| v Members   2    +--------------------------------+
|   @alice         |   [ composer ]                 |
|   @bob           |                                |
+------------------+--------------------------------+
```

## Current state

* `community_ui::CommunityPanel` renders its own 220px left column (channels +
  members) next to the timeline.
* `Sidebar` renders the Recents/Chats/Communities tabs, and `Sidebar::open_community`
  records the community as recent and asks `CommunityRegistry` to emit
  `CommunityEvent::Open`, which `Workspace` turns into a center dock panel.
* `Community` already folds channels (`state().channels`), members, the control
  roster (`control().roles`), the community icon, and the banner (`metadata.banner`).

## Plan

### 1. Backend: community crate

`crates/community/src/community.rs`

* Add `CommunityEvent::Close(CommunityId)` and `CommunityEvent::Channel(CommunityId, ChannelId)`.
* Add `active: Option<ChannelId>` to `Community`, with:
  * `pub fn active_channel(&self) -> Option<ChannelId>` — the selected channel,
    defaulting to the first one.
  * `pub fn set_active_channel(&mut self, ChannelId, &mut Context<Self>)` — stores
    it and emits `Channel`.
* Add `banner: Option<PathBuf>` resolved from `metadata.banner` the same way the
  icon already is, exposed as `pub fn banner(&self) -> Option<PathBuf>`.

`crates/community/src/lib.rs`

* Add `CommunityRegistry::emit_close(&mut self, CommunityId, &mut Window, &mut Context<Self>)`,
  mirroring `emit_community`, emitting `Close`.

`crates/community/src/sync.rs`

* Rename `resolve_icon` to `resolve_image` (it takes any `ImageRef`).

### 2. `community_ui`: panel becomes timeline-only

* Drop the internal channel/member column and its helpers.
* Resolve the shown channel from `Community::active_channel` on every load and
  reload, and subscribe to `CommunityEvent::Channel` to follow sidebar clicks
  (deferred, because the community is being updated when it emits).

### 3. `workspace`: sidebar renders the community

`crates/workspace/src/sidebar/mod.rs`

* New state: `community: Option<WeakEntity<Community>>` and three section flags.
* `open_community` also sets `self.community`; new `close_community` clears it and
  asks the registry to emit `Close`.
* `render` picks between the existing tabs and `render_community`.
* `render_user` keeps the account button as usual and adds a back button on the
  right when a community is open.
* `render_community` pins the optional banner image and scrolls everything below
  it: collapsible Channels, Admins (members where `control().roles.is_staff`), and
  Members sections. Every row is a `TreeRow` — the component the sidebar's own
  lists use — so height, padding, typography, hover, and selection match, and each
  row is wrapped `flex_shrink_0` so the list scrolls instead of squashing.
* Observe the registry so member/roster changes re-render the lists.

`crates/workspace/src/lib.rs`

* Remember the opened `CommunityPanel` so `CommunityEvent::Close` can close it:
  activate it, focus the group, then dispatch `ClosePanel`.

### 4. `community`: a list refresh keeps entities

`CommunityRegistry::track` used to rebuild every community, so an open panel and a
browsing sidebar went stale on any list signal — and `sync::load` merges join
material into a loaded `CommunityState`, so the state really can change. `track`
now keeps the entity for an id it still tracks and hands it the new state through
`Community::adopt`, which also cancels a fold in flight that would otherwise write
the pre-adoption state back. Observers move to a
`HashMap<CommunityId, Subscription>` so the survivors keep theirs, and the ids the
list dropped lose their observer and their synced key.

## Out of scope

Moderation, invites, community management, channel icons, unread badges, member
search, and persisting the selected channel across restarts.