T22: Collaboration

- presence: avatars ringed in each peer's color with where they are, press to
  follow; a breathing live dot; follow mode frames the view and stops on Escape
- remote cursors and selections painted over a field from its own layout
- comment threads with replies, reactions and resolve; a marker that opens a
  thread in a bubble; a sidebar of open and resolved threads
- annotations washed in their authors' colors, notes on hover
- suggestion mode: edits become suggestions word by word; track changes shows
  them struck and underlined, to accept or reject
- sharing: invite by email with checked chips, roles, the access list and who
  the link lets in, in one dialog
- primitives::hold_focus gives a mode its own focus, shared with ZenMode
- AvatarGroup's tuck and count shared; Avatar::ring_color; TagInput::check;
  TextInput::bounds_for_range; Popover::size
- icons: smile-plus, user-plus, message-square-diff
- gallery: the Collaboration page with scripted states

src/primitives/focus.rs

@@ -170,6 +170,46 @@ pub(crate) fn give_back(state: &Entity<Takeover>, window: &mut Window, cx: &mut
     }
 }
 
+/// A mode's own focus, where focus was before the mode took it, and whether the mode was on last frame.
+pub(crate) struct Held {
+    pub focus: FocusHandle,
+    previous: Option<FocusHandle>,
+    was_on: bool,
+}
+
+/// The focus a mode keyed `key` holds: turned on while focus is elsewhere, it takes focus so Escape reaches it.
+pub(crate) fn hold_focus(
+    key: impl Into<ElementId>,
+    on: bool,
+    window: &mut Window,
+    cx: &mut App,
+) -> Entity<Held> {
+    let held = window.use_keyed_state(key, cx, |_, cx| Held {
+        focus: cx.focus_handle(),
+        previous: None,
+        was_on: false,
+    });
+    let (focus, was_on) = (held.read(cx).focus.clone(), held.read(cx).was_on);
+    if on && !was_on && !focus.contains_focused(window, cx) {
+        log::info!("focus: a mode takes it, so Escape reaches it");
+        let previous = window.focused(cx);
+        window.focus(&focus);
+        held.update(cx, |held, _| held.previous = previous);
+    }
+    if on != was_on {
+        held.update(cx, |held, _| held.was_on = on);
+    }
+    held
+}
+
+/// Hands focus back to where it was before the mode took it.
+pub(crate) fn hand_back(held: &Entity<Held>, window: &mut Window, cx: &mut App) {
+    if let Some(previous) = held.update(cx, |held, _| held.previous.take()) {
+        log::info!("focus: handed back after a mode");
+        window.focus(&previous);
+    }
+}
+
 /// A focus handle kept for `id`, a Tab stop while enabled.
 pub(crate) fn tab_stop(
     id: ElementId,