Session switching gets progressively slower as message history grows #17775

Description

Switching between sessions becomes progressively slower the more the user has scrolled up through message history. Each round-trip (switch away, switch back) re-fetches an ever-growing number of messages from scratch.

Root cause: meta.limit in sync.tsx grows via loadMore() as the user scrolls up (200 → 400 → 600 → ...). When switching back to a session after the 15s prefetch TTL expires, a force-refresh reuses this inflated limit as the server fetch size — re-downloading the entire browsing history in a single request instead of just the latest page.

Example: user scrolls up through 1000 messages, switches away for 15s, switches back → server is asked for limit=1000 in one request. Next scroll-up grows it to 1200, next switch re-fetches 1200, etc.

Steps to reproduce

1. Open a session with many messages (100+)
2. Scroll up several times to load older history (each scroll triggers loadMore, growing the limit)
3. Switch to a different session
4. Wait 15 seconds (prefetch TTL)
5. Switch back to the original session
6. Observe: network request fetches all previously-loaded messages, not just the latest page
7. Repeat steps 2-6 — each cycle gets slower

OpenCode version

1.2.26

Operating System

Any