When transcript virtualization is disabled (the #4325 opt-out,
_virtualizeTranscript===false), renderMessages() renders every row with no
windowing and never runs the virtualized measure pass
(_updateMessageVirtualMeasurements early-returns when !virtualized). Under
@media (pointer: coarse), .msg-row[data-role="user"] carries
content-visibility: auto; contain-intrinsic-size: auto 96px. Every rebuild does
inner.innerHTML='' then recreates rows as fresh elements, so a fresh off-screen
tall user row (a long paste measuring thousands of px) reserves only the flat
estimate instead of its real height. scrollHeight shrinks by (realHeight -
estimate), the browser force-clamps scrollTop, and the viewport jumps backward
(a browser clamp, JS=none, so scrollTop-write compensation cannot catch it).
#5638 fixed this for the virtualized wipe-and-rebuild path but left the
non-virtualized full-rebuild path uncovered.
Fix, three coordinated parts:
- _estimateUserRowIntrinsicHeight weights CJK / full-width characters as ~2
columns (they wrap at ~24 chars/line, not 48), so a CJK paste reserves close
to its real height even before it is ever measured.
- _applyUserRowIntrinsicHeight reserves max(remembered, estimate): a remembered
height can be a partial paint (a row taller than the viewport only paints its
intersecting slice under content-visibility:auto), so the estimate floors it.
- _rememberRenderedUserRowIntrinsicHeights, called pre-wipe inside
renderMessages, reads the still-laid-out rows' real heights and persists them
keyed by session-relative index, only for rows within the viewport (a fully
off-screen never-painted row reports its collapsed reserve and must not poison
the map), floored at the estimate.
Desktop rests at content-visibility:visible so intrinsic-size is inert there;
verified no behavior change with pointer:fine.
Adds tests/test_issue5744_nonvirtual_userrow_collapse_jumpback.py (7
mutation-checked node-harness tests). Existing #5637/#5638 suites and the
render/virtualization suites pass.
