Fix @mention search performance and refactor search functionality

[ZhangHanDong]

Fixed issue #506

This is a remastered version of the PR #550 .

• Add a lightweight cpu_worker thread that stays alive for the app lifetime and runs CPU-bound jobs (currently the room-member search); App::handle_startup initializes it and widgets enqueue work via cpu_worker::spawn_cpu_job.
• Rework MentionableTextInput into an explicit state machine with search IDs, cancellation tokens, loading/empty UI states, and streaming updates that arrive over the background worker channel.
• Replace the old MatrixRequest::SearchRoomMembers path: sliding_sync now fetches joined-member lists directly, emits RoomMembersListFetched, and RoomScreen caches those members plus precomputed sort data for downstream widgets.
• Refactor room/member_search.rs to support batched streaming, cancellation, richer match heuristics, optional precomputed sort reuse, and add unit tests for the new helpers.

There are three related issues that still need to be fixed.

• #615
• #617

[kevinaboos]

@alanpoon & @tyreseluo, can you please give this a review before I dig in? Thanks.

[tyreseluo]

@alanpoon & @tyreseluo, can you please give this a review before I dig in? Thanks.

yep, i will.

[ZhangHanDong]

Review suggestions: • Select a large room (thousands or tens of thousands of members), @ a user, and search. • Continue testing in rooms with more than 50 members. • Continue testing in rooms with fewer than 50 members. • Close large/small rooms, reopen them, and continue testing. • Logout, then login again, and rerun the above four steps.

[ZhangHanDong]

@kevinaboos

Current PR progress:

1. Fix the issue where remote sync of members does not work after logout-login - Previously, after logging out and then logging back in, remote synchronization of room members failed to trigger properly - The relevant state reset logic has been corrected
2. Remove the behavior of automatically deleting the local members cache after closing a tab - This behavior is considered unnecessary and harms the user experience - In the future, a setting can be added to manually clear local cached data
3. Optimize the member list loading experience - Automatically check local cached members; if a cache exists, render the user list immediately without waiting for remote data - Each room performs remote synchronization only once per application session - During synchronization, show a loading animation at the bottom of the user list; it will disappear automatically after sync completes
4. Fix the issue where the cursor disappears after clicking to select a mention user with the mouse - Previously, after clicking to select a user in the mention popup, the cursor would blink and then disappear, making it impossible to continue typing - Resolved by retrying the focus restoration mechanism in draw_walk
5. Fix duplicate room tabs after app restart or logout-login - Previously, restoring rooms could create duplicate tabs - Avoided by looking up existing tabs based on room_id

Remote sync room_members trigger conditions:

Remote member synchronization (SyncRoomMemberList) is triggered in the following cases:

• First time opening a room after the application starts
• Login again after Logout
• Switching to a new room that has never been loaded

Note: Within the same application session, closing a room tab and reopening it will not trigger remote synchronization again.

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The current code still needs another review; functional testing can proceed first.

[ZhangHanDong]

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Summary

This PR comprehensively refactors the @mention member search functionality to address performance issues in large rooms (#506).

Key Architectural Changes

1. CPU Worker Module - A lightweight background thread handles CPU-bound member search operations, keeping the UI responsive during searches in large rooms.

2. State Machine Design - MentionableTextInput now uses an explicit state machine (MentionSearchState) with four states: Idle, WaitingForMembers, Searching, and JustCancelled. This replaces multiple boolean flags and reduces race conditions.

3. Streaming Search Results - Results are sent via MPSC channel in batches of 10, enabling progressive UI updates. Each search has a unique ID and cancellation token support.

4. Precomputed Sort Data - PrecomputedMemberSort caches sort keys and indices, making empty searches O(1) and avoiding repeated computation.

5. Local-First Rendering - Member lists render immediately from local cache while remote sync continues in background.

Bug Fixes

• @room in DMs: Block @room mentions in direct message rooms
• Duplicate Tabs: Fix duplicate room tabs on restore by using find_open_room_live_id() lookup
• Focus Loss: Restore text input focus after mention selection via pending_draw_focus_restore
• Logout/Login: Clear Matrix SDK database on logout to prevent stale data

Duplicate Tabs

Security Fix

• Add validate_path_within_app_data() to prevent arbitrary directory deletion when clearing app state.

The database path from session files is now validated to be strictly within the app data directory before deletion.

Documentation

• Added comprehensive module documentation with ASCII diagrams for state machine and background thread flow
• Documented duplicate tab prevention mechanism
• Documented database cleanup rationale and security measures

Test Plan

• [ ] Verify @mention search is responsive in large rooms (1000+ members)
• [ ] Verify @room option does not appear in direct messages
• [ ] Verify no duplicate tabs when clicking already-open rooms
• [ ] Verify focus remains in text input after selecting a mention
• [ ] Verify logout and re-login shows fresh data without stale cache
• [ ] Verify ESC cancels mention search and prevents immediate re-trigger