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

AGENTS.md

@@ -113,7 +113,8 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - `ScrollHandle` keeps its children's bounds unscrolled: a child paints at its bounds plus the offset. A press on a scrolled child subtracts both, and an offset that brings a child to a place is set whole, not added (`documents::find::reveal`).
 - A find box is `editor::FindWidget`, over code, a terminal or a document's pages. It shows the replace fold and the option toggles only when the owner handles them.
 - Taffy 0.9 hands negative free space to auto margins and centering alike, so a centered child wider than its scroll box starts past the scroll's reach. Center only what fits the measured box (`documents::DocumentViewer`).
-- A mode that leaves on Escape keeps its own focus handle: turned on while focus is elsewhere, it takes focus so Escape reaches it, and hands focus back when it leaves (`documents::ZenMode`).
+- A mode that leaves on Escape holds its own focus through `primitives::hold_focus`: turned on while focus is elsewhere, it takes focus so Escape reaches it, and `hand_back` returns it (`documents::ZenMode`, `collab::FollowMode`).
+- A mark over a text field reads the field at paint time, from a canvas laid over it (`collab::RemoteCursor`): by then `TextInput::bounds_for` holds this frame's layout, and a still page never renders again to catch up.
 - A callback field holds a named `type` alias; clippy's type_complexity rejects `Option<Rc<dyn Fn(..)>>`. A chapter shares one alias per shape.
 - A component that fills its container says `w_full` on its root. As a window's root, or in a flex row, it would shrink to its text's narrowest width.
 - A field's `Highlight` sets color, wash, weight, slant and strike per span; measuring and painting share one set of runs, so wrapped heights match.