## Why
Pressing `Home` while older transcript history is loading leaves a pending jump to the beginning. Subsequent navigation should supersede that jump so arriving history does not override the user's new reading position.
## What changed
Cancel the pending jump when scrolling, jumping to the latest entry, or navigating prompts in the read-only transcript viewer. Keep the in-flight history page by switching from `LoadingBeginning` to `LoadingOlder`. Handle navigation before the first draw as well as after rendering, while preserving pending jumps during passive highlight restoration.
## Testing
Add regression coverage for navigation after `Home` before and after the first draw, checking that arriving older history preserves the expected viewport. Extend prompt backtracking tests to verify that navigation clears the pending jump.
GitOrigin-RevId: 438c3289afee2719655b11dbc43a3ccb006a5fa3
392f56a611