feat(threads): thread drawer, thread browser, live reply counts, and unread badges

[Just-Insane]

📝 Docs: Already live at Threads — Sable Docs — no separate docs PR needed.

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Description

Expands Matrix thread support with a full thread drawer, server-side thread browser, live reply counts, unread badges, and several UX fixes.

ThreadDrawer

• Slide-in drawer for reading and replying in threads without leaving the main room
• Thread root message shown at top with full reply history below
• Up-arrow edit-last-message: pressing ↑ in the thread input edits the user's last own message in that thread
• Reactions add/remove trigger immediate re-renders without manual refresh (handles the fact that reaction events carry m.relates_to.event_id not threadRootId directly — fetches target event to check thread membership)
• Thread root message box sized to content with maxHeight: 200px to avoid dead whitespace on short messages

ThreadBrowser

• Lists all threads in the room; calls room.fetchRoomThreads() on mount to load threads beyond the local timeline window
• Load-more button when the server has additional thread pages
• Each thread preview shows sender avatar, display name, reply count, and last message body with ellipsis truncation
• Jump button navigates to the thread root in the main timeline (via useRoomNavigate) instead of re-opening the drawer
• Thread unread badges on each preview item via room.getThreadUnreadNotificationCount()

ThreadReplyChip (main timeline)

• Shows unread badge for threads with unseen replies via room.getThreadUnreadNotificationCount()

Thread button (RoomViewHeader)

• If a thread drawer is open, button closes the drawer and opens the thread browser (allows going from a single thread to the full list)
• If no thread is open, button toggles the thread browser as before
• aria-pressed reflects both thread browser state and open thread drawer

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does)
• Fully AI generated (explain what all the generated code does in moderate detail)

ThreadDrawer/ThreadBrowser/thread-chip iterations were partially AI assisted; I reviewed and adjusted logic for timeline processing, thread loading, and event rendering behavior.

[Copilot]

Pull request overview

Adds end-to-end Matrix thread support across the room timeline: a thread drawer for reading/replying in-thread, a thread browser listing room threads (with pagination), and live reply/unread indicators, backed by Matrix SDK threadSupport.

Changes:

• Enable SDK thread tracking by passing threadSupport: true to startClient (classic + sliding sync).
• Introduce shared edit-mode hook (useMessageEdit) and wire it through RoomTimeline/timeline actions and ThreadDrawer.
• Implement thread UI primitives: ThreadDrawer ported to the shared timeline rendering pipeline, ThreadBrowser backed by /threads + pagination/backfill, and live reply/unread badges on timeline thread chips.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/client/initMatrix.ts	Enables Matrix SDK thread support in both classic and sliding sync start paths.
src/app/hooks/useMessageEdit.ts	New hook to centralize edit-mode state/reset/focus behavior for composers.
src/app/hooks/timeline/useTimelineEventRenderer.tsx	Adds live thread reply/unread chip updates; supports suppressing thread chip/reply headers inside drawer.
src/app/hooks/timeline/useTimelineActions.ts	Refactors timeline actions to accept shared handleEdit instead of owning edit state.
src/app/hooks/timeline/useProcessedTimeline.ts	Adds skipThreadFilter to allow processing thread timelines without filtering out replies.
src/app/features/room/ThreadDrawer.tsx	Reworks thread drawer to use shared timeline processing/rendering; adds sequential loading/backfill and edit-last-message support.
src/app/features/room/ThreadBrowser.tsx	Adds server-backed thread list with load-more pagination, root-event backfill, and unread badges.
src/app/features/room/RoomTimeline.tsx	Adopts useMessageEdit and passes handleEdit into timeline actions.
src/app/features/common-settings/developer-tools/DevelopTools.tsx	Adds thread diagnostics + “fetch from server” action for debugging thread state.
.changeset/feat-thread-enhancements.md	Declares a minor release note for thread support features.

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[7w1]

Proper threads! Yay! only thing I noticed is some styling with scrollbars in the thread drawer:

[7w1]

Wait why do we need a button? Can’t be just load more when the user scrolls up like a normal timeline? I must be getting something confused lol.
Also, scroll bar on main drawer still needs styling.