Category: Message Display / Threading

Summary: Add an optional tree-style threading view that visually displays parent-child relationships between messages in a conversation thread, allowing users to see exactly which email a specific reply is referring to.

Current Behavior: Proton Mail's conversation grouping collapses multiple messages from the same thread into a single expandable entry, displaying them chronologically when expanded. However, this presents replies in a flat, sequential list without visual indication of which message each reply is responding to. In long or multi-party conversations where users branch off different topics, it can be unclear which prior message triggered a particular response.

Proposed Enhancement: Introduce an optional "thread view" mode that renders messages as a collapsible tree structure, indenting replies beneath their parent messages. Key features:
* Visual hierarchy: Indented branches showing parent-child relationships
* Expand/collapse: Ability to collapse sub-branches to reduce clutter
* Toggle option: Allow users to switch between current flat list and tree view
* Works with existing grouping: Compatible with current conversation collapsing functionality

Use Case / Benefits
1. Complex threads: In multi-topic conversations where different participants reply to different messages within the same thread, the tree view makes dependencies explicit
2. Long-running discussions: Easier to follow conversational flow in threads spanning many messages or weeks/months
3. Audit trails: Helpful for compliance, documentation, or understanding decision-making sequences
4. Cross-client parity: Matches behavior available in desktop clients like Thunderbird and Apple Mail, reducing friction for users who rely on both Proton Mail and traditional email workflows

Technical Feasibility Note: Since Proton Mail reads References and In-Reply-To headers for conversation grouping, the data needed to reconstruct parent-child relationships is already available in message metadata. Implementing tree visualization would primarily be a UI rendering enhancement rather than requiring new backend infrastructure.