TF-4274 docs(adr): add ADR-0070 regarding sync strategy for disappearing emails

docs/adr/0070-sync-strategy-disappearing-emails.md

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+# 0070 - Synchronization Strategy for Disappearing Emails (Cache Invalidation & Query Logic)
+
+## Status
+
+Accepted
+
+## Context
+
+We have identified a recurring synchronization issue where emails disappear from the user's Inbox view despite still existing on the server. This issue affects the reliability of the application and confuses users.
+
+### The Scenario (Observed Behavior)
+
+The issue typically manifests in the following sequence, as reported in [Issue #4274](https://github.com/linagora/tmail-flutter/issues/4274):
+
+1. **Day 1:** A user receives "Email A" in their Inbox and reads it.
+2. **Day 2:** The user opens the application, but "Email A" is no longer visible in the mail list.
+3. **Troubleshooting attempts:**
+* Switching mailboxes (e.g., to Sent and back to Inbox) does not restore the email.
+* Clicking the standard UI "Refresh" button has no effect.
+* **Crucially:** The email *can* be found using the Search function (proving data existence).
+
+
+4. **Resolution:** The user is forced to **Logout and Login**, which magically restores the email to the list.
+
+### Root Cause Analysis
+
+The behavior indicates a desynchronization between the local application state (Cache/UI) and the server. The fact that Logout/Login resolves the issue confirms that the local session cache holds stale or corrupted list data that standard soft-refreshes fail to invalidate.
+
+## Decision
+
+To resolve this, we are standardizing the **Cache Invalidation** triggers and introducing an **Environment-Controlled Query Strategy** for mailbox switching.
+
+### 1. Standardization of Cache Invalidation Triggers
+
+We define specific "Hard Actions" that must strictly trigger a complete cache clearance to ensure data freshness. We move away from "soft updates" for these specific interactions to guarantee the UI reflects the server state.
+
+* **F5 (Browser Refresh):**
+* **Action:** Triggers `clear cache`  Reloads application.
+
+
+* **Logout:**
+* **Action:** Triggers `clear cache` (Session wipe)  Redirects to login.
+
+
+* **UI Refresh Button:**
+* **Action:** Triggers `clear cache` explicitly for the current view  Re-fetches list from server.
+* *Rationale:* Previously, this might have only requested a delta update. Now it forces a clean slate fetch for the current list.
+
+
+
+### 2. Configurable Mailbox Switching Logic
+
+The "Switch Mailbox" action triggers a data query. To prevent the specific conflict or race condition causing the disappearance of emails, we are introducing a feature flag to control the query behavior.
+
+We will introduce a new environment variable `FORCE_EMAIL_QUERY`.
+
+**Configuration:**
+In the application's environment file (`env.file`):
+
+```properties
+# Controls the query strategy when switching mailboxes
+FORCE_EMAIL_QUERY=false
+
+```
+
+**Logic Flow:**
+
+* When a user switches mailboxes, the system checks the `FORCE_EMAIL_QUERY` flag.
+* **Set to `false` (The Fix):** The application will utilize the optimized query logic (relying on the refined cache/fetch strategy) rather than forcing a heavy query that may be contributing to the state inconsistency observed in the issue.
+
+## Consequences
+
+### Positive
+
+1. **Reliability:** Directly addresses the "disappearing email" bug, ensuring users always see their actual messages.
+2. **User Experience:** Eliminates the need for users to perform a disruptive Logout/Login cycle to restore their view.
+3. **Maintainability:** By externalizing the logic to an environment variable (`FORCE_EMAIL_QUERY`), we allow DevOps/Developers to toggle query behaviors in different environments without altering source code.
+4. **Clarity:** clearly defines what "Refresh" implies for the system (Clear Cache vs. Delta Update).
+
+### Negative
+
+1. **Performance Overhead:** Forcing a `clear cache` on the UI Refresh button is more resource-intensive than a delta update, potentially leading to slightly longer loading spinners for users with large mailboxes.
+2. **Configuration Risk:** Incorrectly setting `FORCE_EMAIL_QUERY` in production environments could revert the fix or alter application behavior unexpectedly.
+
+## Implementation Steps
+
+1. **Event Handling Update:**
+* Refactor the **Refresh Button**, **F5**, and **Logout** handlers to ensure they explicitly call the `clearCache()` method before fetching new data.
+
+
+2. **Environment Configuration:**
+* Add `FORCE_EMAIL_QUERY` to the default `env.file` and set it to `false`.
+* Update the `AppConfig` or `Environment` class to read this variable.
+
+
+3. **Mailbox Logic Update:**
+* Modify the `switchMailbox` function to check the boolean value of `FORCE_EMAIL_QUERY` and apply the corresponding query logic.
+
+
+4. **Verification:**
+* Reproduce the scenario (Read email -> Simulate wait/stale state -> Verify visibility).
+
+
+
+## References
+
+* **Github Issue:** [linagora/tmail-flutter#4274](https://github.com/linagora/tmail-flutter/issues/4274)