TUI shows "N more lines above · still updating" and stops refreshing, leaving output incomplete #478

Reported by @GoldTest in #426 after upgrading to 0.6.5, where the original jump-to-top problem was fixed.

Symptom: The terminal occasionally stays on "750 more lines above · still updating". After several more interactions the content never updates, so the terminal output is shown incomplete. The reporter attached two screenshots in #426.

Missing info: terminal/host and OS, MCode version, rough steps before it got stuck, and logs from around that time.

Status: Not reproduced yet. Needs more details from the reporter.