Session switch causes ~170ms blank flash and Suspense splash screen

[BYK]

Describe the bug

Session switching has three compounding responsiveness issues:

1. deferRender mechanism blanks the review panel and composer for one animation frame on every session switch, and a 140ms setTimeout in session-composer-region stacks on top, causing a ~170ms gap where the composer is invisible.
2. sessionSync createResource JSX read — {sessionSync() ?? ""} is read in JSX, triggering the app-level <Suspense> boundary (splash screen) on every session switch while the sync Promise is pending. The resource returns void and contributes nothing to the DOM.
3. messagesReady gate blocks rendering until messages have loaded from the API, even though the session exists in the session list and could be shown immediately with a loading spinner.

Steps to reproduce

1. Open the web UI
2. Switch between sessions rapidly
3. Observe blank flashes, splash screen flickers, and delayed composer appearance

Expected behavior

Session switching should be instant — show the session frame immediately with cached data and load messages in the background with a spinner.

System info

• OpenCode version: v1.15.3
• OS: Linux

[github-actions]

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

• Session navigation blocks UI when switching between large conversations #19793: Session navigation blocks UI when switching between large conversations — same web app, same session switching context, overlapping root cause (rendering/staging during session switch)
• [Bug] Desktop v1.3.0 UI Regression: Sessions flash and disappear; workspace switching leads to semi-blank screen #18933: [Bug] Desktop v1.3.0 UI Regression: Sessions flash and disappear; workspace switching leads to semi-blank screen — similar symptom of blank/flash on session switch

If these cover a different aspect of the problem, please clarify how this issue differs so we can keep it open.

[BYK]

Thanks — #19793 describes the same session switching latency. PR #27912 addresses this with three fixes: removing deferRender, preventing Suspense flash, and instant rendering with cached data.