Forte Reactions — Step 2: Feature Description

Problem

Forte's board has no way to Like a thread or Flag a post; classic already supports both, but the classic Like/Flag implementation assumes it's only ever showing reactions for one thread at a time.

User Stories

• As a Forte board-view reader, I want to Like a thread and see my Like reflected immediately, so I can react the same way I would in classic.

• As a Forte board-view reader, I want to Flag a post (including a thread's root post), so problem content gets the same signal it would in classic.

• As a returning Forte board-view reader, I want threads/posts I've already reacted to to show that state (e.g. "Liked" disabled), so I don't wonder whether my earlier reaction took.

Core Requirements

• Like (thread-level, apply_thread_tag) and Flag (post-level, apply_post_tag — including on a thread's root post) both work from Forte's board, using the existing endpoints and thread_reactions.js unchanged in behavior (optimistic UI, pending state, error feedback).

• thread_reactions.js's thread-level binding is extended to bind every thread's Like button on the board (per approved Step 1, Option A), not just one.

• Reaction buttons trigger the same lazy identity-preparation flow already wired for signed replies (window.__forumBrowserIdentity, loaded via lazy_compose_signing.js) rather than requiring a separate load path — a reader clicking Like/Flag before ever touching the reply composer still gets prompted the same way.

• The board reflects each viewer's existing reaction state (already-Liked threads show "Liked" disabled, already-Flagged posts show "Flagged" disabled) on page load, matching classic's behavior — not a stripped-down, state-blind version.

• No changes to classic's own thread page, its reaction buttons, or the underlying tag-writing logic.

Shared Component Inventory

• /api/apply_thread_tag, /api/apply_post_tag — canonical endpoints. Reused unchanged.

• thread_reactions.js — canonical fetch/optimistic-UI/identity logic. Extended (thread-level binding only, per Step 1); post-level binding already supports multiple roots and needs no change.

• Reaction button markup (thread_root_card.php / post_card.php) — not reused directly (Forte's reply tree renders posts via a raw-HTML builder, not shared partials); new Forte-styled buttons carry the same data-action/data-tag/data-post-id contract thread_reactions.js already expects.

• Per-viewer reaction state lookup (viewerHasThreadTag, viewerPostTagsForPosts) — the post-level helper already batches across a list of post IDs (used today for one thread's posts); reused, extended to batch across every thread/post on the board at once, mirroring the existing bulk-fetch pattern forte_board_reply_tree already established for reply trees.

Simple User Flow

1. Reader selects a thread on the board and clicks Like (or Flag on any visible post).

2. If needed, the same identity-preparation flow already used for signed replies runs.

3. The button shows pending, then confirmed state; the thread's score or post's flag state updates in place, no page reload.

4. On a later visit, already-reacted-to items show their Liked/Flagged state immediately.

Success Criteria

• Like and Flag both work from Forte's board and match classic's outcome exactly (same endpoints, same resulting tag records).

• Multiple threads' Like buttons on the same board page work independently and correctly.

• Existing viewer reaction state is visible on page load, not just after a fresh click.

• Classic's thread page and its reactions are completely unaffected.