Streaming content duplication during LLM streaming in desktop/web UI

[praxstack]

Bug Description

During LLM streaming in the desktop UI (and web app), text content appears duplicated/repeated. After quit+reopen, content displays correctly (loaded from DB).

Steps to Reproduce

1. Open the opencode desktop app or web UI
2. Send a message to trigger LLM streaming
3. Observe the streaming text output — content appears repeated/duplicated
4. Quit and reopen the app — content now displays correctly

Expected Behavior

Streaming text should appear incrementally without duplication.

Actual Behavior

Text content is duplicated during streaming. For example, instead of showing Hello world, the UI shows Hello worldHello world.

Root Cause

In global-sdk.tsx, SSE events are batched in 16ms windows with coalescing. When message.part.updated events coalesce within a single batch (text-end replaces text-start at the same queue index), stale message.part.delta events remain in the queue and get applied on top of the authoritative full text.

Environment

• Platform: macOS (desktop app via Tauri), also affects web UI
• Observed on latest dev branch

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Desktop App: UI renders massively duplicated content when streaming responses with code blocks #14560: Same author, same issue — "Desktop App: UI renders massively duplicated content when streaming responses with code blocks" — covers the same streaming duplication behavior in the Desktop App/web UI
• Some AI text is printed twice or more times #14623: "Some AI text is printed twice or more times" — same symptom of duplicated streaming output

Please check if your issue is already covered by one of the above. If so, consider closing this one and adding any new details to the existing issue instead.