Virtualization (#500/#4214) caused a scroll-up flicker on long sessions with
tall tool-call rows (variable-height anchor oscillation), distinct from the
stream-completion jump fixed in #4325. Per maintainer decision, flip the
'Virtualize long transcripts' preference to experimental/opt-IN (default OFF)
for everyone until the scroll behavior is reliable.
- api/config.py: virtualize_transcript default True->False; add
virtualize_transcript_optin marker (both bool-allowlisted); load_settings
force-off migration — a stored true WITHOUT the post-flip opt-in marker is
reset to off, so 100% of existing users land on off; an explicit opt-in
(marker present) is honored.
- boot.js / panels.js: default-off semantics (===true), persist the opt-in
marker on enable, checkbox not pre-checked.
- index.html / i18n.js (en): relabel experimental + flicker warning.
- Tests: rewrite the #4325 toggle assertions for the opt-in contract + 4 new
migration tests (unset/stale-true/explicit-optin/marker-without-true).
Phase B (root-cause scroll-stability) tracked separately. Thanks @b3nw.
nesquena-hermes
5a8d24c936