ADR-71 CollapseThread

docs/adr/0071-collapse-threads-in-email-query.md

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+# 0071 - Enable collapseThreads in Email/query
+
+Date: 2026-03-12
+
+## Status
+
+Proposed
+
+## Context
+
+JMAP `Email/query` supports `collapseThreads` (RFC 8621 §4.4). When `true`, server returns only the **latest email per thread**, avoiding duplicate rows in the mail list.
+
+Currently, our app does **not** set `collapseThreads`. When thread is enabled, users see multiple emails from the same thread as separate rows.
+
+### Two Query Paths (see ADR-0070)
+
+| Path | `FORCE_EMAIL_QUERY` | Method | Cache |
+|------|---------------------|--------|-------|
+| Force query | `true` | `forceQueryAllEmailsForWeb()` | No local cache — server only |
+| Cache-first | `false` | `getAllEmail()` | Local DB first, then sync |
+
+### Problem: Cache Inconsistency on Toggle
+
+On the cache-first path, the local DB reflects the `collapseThreads` mode it was built under. When thread mode changes:
+
+- **OFF → ON**: Cache has individual emails (5 rows) but server now returns collapsed (3 rows) → stale first yield
+- **ON → OFF**: Cache has collapsed emails (3 rows) but server now returns all individuals (5 rows) → missing emails in first yield
+
+Force-query path has **no problem** — always queries server directly.
+
+## Decision
+
+### 1. Set `collapseThreads: true` in Email/query When Thread Is Enabled
+
+In `MailAPIMixin.fetchAllEmail()` and `ThreadAPI.searchEmails()`, pass `collapseThreads: true` when thread is enabled.
+
+### 2. Clear Email Cache on Thread Setting Toggle
+
+Reuse the existing thread-enabled setting. **No new flag needed.** When thread is toggled (enable ↔ disable), clear email cache immediately in the toggle handler. Next `getAllEmail()` will rebuild cache under correct mode.
+
+### 3. No Change for Force-Query Path
+
+Already server-only. Just pass `collapseThreads: true` in the query.
+
+## Consequences
+
+**Positive**: Correct thread display, reduced data per page, spec-compliant.
+
+**Negative**: One-time full reload on each toggle; toggle handler coupled with cache management.
+
+## Implementation Steps
+
+1. Add `collapseThreads` to `QueryEmailMethod` in `MailAPIMixin.fetchAllEmail()` and `ThreadAPI.searchEmails()`
+2. Pass thread-enabled status from Settings to query builder
+3. Clear email cache in thread setting toggle handler
+4. Update `GetEmailsInMailboxInteractor` and related interactors to propagate thread status
+
+## References
+
+- [RFC 8621 §4.4](https://www.rfc-editor.org/rfc/rfc8621#section-4.4)
+- [ADR-0070](./0070-sync-strategy-disappearing-emails.md)
+- `lib/features/base/mixin/mail_api_mixin.dart` — `fetchAllEmail()`
+- `lib/features/thread/data/repository/thread_repository_impl.dart` — `getAllEmail()`
+- `lib/features/thread/domain/usecases/get_emails_in_mailbox_interactor.dart`