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Virtualization and precise deep-link landing have fundamentally opposed
contracts: virtualization uses estimated heights for off-screen items,
deep-link needs real heights for everything above the target. Three
prior fix attempts (initial scrollToIndex race, settle-by-silence
observer, 3-pass cooperative scroll) all tried to satisfy both in one
path and none fully stabilized — code/image/mermaid-heavy comments
kept drifting the target after first landing.
Split by user intent instead:
- highlightCommentId set (user came from inbox to read a specific
comment) -> render flat. Every comment mounts, every height is real,
the target id is in the DOM the instant the effect runs. Native
document.getElementById + el.scrollIntoView({block:'center'}) is
semantically identical to a native <a href="#comment-X"> anchor.
- otherwise -> Virtuoso. Browsing mode keeps the first-paint perf win
from #2413 on long timelines.
Deep-link effect collapses to ~22 lines, matching the pre-virtualization
implementation. A shared renderItem function keeps both render modes
consistent. Removes: bootstrapRef, three-pass scrollToIndex effect,
overflow-anchor:none, scrollPaddingTop on container, scroll-margin-top
on every comment wrapper, virtuosoRef + VirtuosoHandle, initialItemCount
prop, useLayoutEffect.
Mermaid gets a 280px skeleton (web.dev CLS guidance) plus a
sessionStorage layout cache keyed by chart-text hash, so the 0px ->
real-height shift no longer drifts the surrounding layout — useful for
both render modes, deep-link or browsing. Pattern matches ant-design/x
#1497 which fixes the same Mermaid drift in their own stack.
Auto-expand a folded resolved thread when the deep-link target is a
reply inside it; without this the target reply stays collapsed and the
user sees only the resolved-bar.
Net: +131 / -245 in issue-detail.tsx. Tests added for the
resolved-thread-reply auto-expand path.
Known follow-ups:
- <ReadonlyImage> aspect-ratio for image CLS (same class as Mermaid).
- Layout heisenbug (page width "abnormal" without devtools open) is
orthogonal to deep-link and survives this PR; needs separate triage.
- 500+ comment cold mount in deep-link mode pays full markdown+lowlight
cost; GitHub takes the same hit and we accept it.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>