Hermes TUI: transcript scroll can blank after rows leave the full-render tail

Summary

In hermes --tui, scrolling back through a long transcript can show blank regions or skip older content after the conversation continues for several more turns.

The likely issue is virtual-row height cache drift: a row can be measured while it is still in the recent full-render tail, then later render much shorter after it leaves the tail and switches to bounded-history rendering.

Reproduction

1. Start hermes --tui.
2. Build a long conversation that includes at least one long assistant response.
3. Continue for several more turns so older rows leave the last FULL_RENDER_TAIL_ITEMS rows.
4. Scroll back through transcript history.

Expected

Older transcript content remains visible and scrollable. Virtual-scroll offsets continue to match the actual rendered row heights as rows move from full-tail rendering to bounded-history rendering.

Actual

Scrolling back can land in blank spacer regions or skip older content, while recent tail content remains visible.

Root cause

Rows inside FULL_RENDER_TAIL_ITEMS render fully. Older assistant rows render through bounded history via limitHistoryRender / boundedHistoryRenderText().

useMainApp.ts builds virtual row keys from message identity/content only, while useVirtualHistory.ts caches measured/estimated heights by that key. A long assistant row can therefore be cached at its full-tail height, then later leave the tail and render much shorter without getting a new virtual-height identity.

That stale full-height measurement makes the virtual offset map overestimate content height, which can push the viewport/clamp into spacer regions.

Relevant paths:

• ui-tui/src/components/appLayout.tsx
• ui-tui/src/app/useMainApp.ts
• ui-tui/src/hooks/useVirtualHistory.ts
• ui-tui/src/lib/virtualHeights.ts
• ui-tui/src/config/limits.ts

Suggested fix

Include the row render mode in the virtual-height identity, or invalidate/re-key cached heights when a row crosses the full-render-tail boundary.

A focused regression should cover a long assistant row measured as a tail row, then aged into bounded-history mode after appending more messages, ensuring the old full-height measurement is not reused.