perf(timeline): gate heavy message render behind useDeferredValue

[tellaho]

Overview

Category: improvement
User Impact: Entering a channel, opening a long thread, or hitting the inbox no longer freezes the app with the OS busy cursor — the message list streams in smoothly instead.

Problem: The timeline renders up to 200 messages synchronously, and each row runs a heavy react-markdown parse. The whole list commits in one blocking pass, pinning the main thread long enough that the OS shows its native busy/spinner cursor. (Shiki is already async — the markdown parse pile-up is the culprit.)

Solution: Phase A of the perf plan — a low-risk concurrency gate, no architectural rewrite. Wrap the message list in useDeferredValue(messages, EMPTY_MESSAGES) so the heavy commit becomes interruptible and streams in on a deferred pass instead of blocking the initial paint. A module-level empty initial value keeps even the first render on channel entry light. All consumers (scroll manager, list-visibility flags, and the TimelineMessageList prop) read the same deferred snapshot so scroll math never tears against the painted DOM — this also closes a latent race where the deep-link scrollIntoView could fire before its target row was committed. This proves the cursor unfreezes before we invest in the larger virtualization effort (Phase B).

File changes

desktop/src/features/messages/ui/MessageTimeline.tsx
Added EMPTY_MESSAGES module-level constant and deferredMessages = useDeferredValue(messages, EMPTY_MESSAGES). Switched the scroll manager, showMessageList/showGenericEmpty flags, and the TimelineMessageList messages prop to read deferredMessages so the deferred snapshot drives everything consistently. Wired the previously-unused pending flag to a subtle opacity-60 dim while a deferred render is in flight, so the list reads as streaming-in rather than frozen.

Reproduction Steps

1. Run the desktop app and open a busy channel (or thread/inbox) with a large backlog of messages.
2. Switch into it. Before this change the native OS busy cursor appears and input locks up during the synchronous render; after, the cursor stays responsive and the list streams in (briefly dimmed while the deferred render lands).
3. Verify the must-keep behaviors still work: the view sticks to the bottom on new messages (sticky-bottom autoscroll), day dividers render between dates, and a jump-to-message deep link scrolls to and highlights the target row.

Notes

• No component-level tests exist for the timeline (suite is lib-only), so the must-keep behaviors were verified by reasoning against the shared deferred snapshot, not automated tests.
• pnpm typecheck ✅ and biome lint ✅ both pass.