The module-level _userRowIntrinsicHeightBySessionIdx cache is keyed by
session-relative index (_messageSessionIndexForRawIdx = _messageSessionIndexBase()
+ rawIdx). The base is 0 for the common non-offset session, so keys collide across
sessions: after switching from a session with tall user rows to one with short
rows, the new session's off-screen rows at colliding keys inherited the previous
session's remembered heights and inflated scrollHeight until re-measured.
Add _clearUserRowIntrinsicHeightCache() and call it from
_clearMessageVirtualHeightCache — the existing per-session height-cache clear that
_resetMessageRenderWindow invokes on session switch — so the new cache shares the
exact same lifetime as _messageVirtualHeightCache. Keys are deleted in place to
keep the const binding stable. Guard the call with typeof so the node harnesses
that extract _clearMessageVirtualHeightCache without this helper keep running.
Adds a mutation-checked regression (test_cache_cleared_on_session_switch_prevents_
stale_height_bleed): a rebuilt row at a colliding key after the clear must fall
back to the content estimate, not the stale remembered height; making the clear a
no-op fails the test.
A virtualization wipe-and-rebuild recreates user rows as fresh DOM elements,
which discards content-visibility:auto's last-remembered size. An off-screen
tall user row (e.g. a long paste) then falls back to the flat
`contain-intrinsic-size: auto 96px` from the stylesheet the instant it is
rebuilt, collapsing scrollHeight by (realHeight - 96px). The browser then either
force-clamps scrollTop (the dTop≈dH jump) or re-anchors to a far row (the dTop≫dH
jump) — both mobile jump-back symptoms trace to this one collapse.
This is layer-scoped to the CSS/content-visibility path and is only active under
`@media (pointer: coarse)` where content-visibility:auto is applied; desktop
rests at content-visibility:visible so the intrinsic-size is ignored (verified:
writing an intrinsic-size on a visible row produces zero scrollHeight/layout
delta), making the change inert on desktop with no platform gate needed.
Fix: remember each user row's measured height keyed by its stable
data-session-msg-idx and re-apply it as an inline contain-intrinsic-size, so a
rebuilt fresh element reserves the real height instead of 96px:
- `_measureMessageVirtualRow` persists + writes the measured height (user rows
only; assistant rows are content-visibility:visible on mobile).
- the user-row build path in `renderMessages` applies the remembered height, or
a content-length estimate (floored at 96px so short rows never regress) before
the row has ever been measured, so even the first fresh-element frame does not
collapse.
Adds tests/test_issue5638_user_row_intrinsic_height_collapse.py (node harness),
mutation-checked: disabling either the measure writeback or the build-time apply
makes the corresponding assertions fail.
allenliang2022