Release: refuse stale-anchor scroll restore during streaming, Android (#5666)

[nesquena-hermes]

Release — refuse stale-anchor scroll restore during streaming, Android (#5666)

Scroll cluster (3 of 4), shipped solo for regression isolation. gate-cert GREEN round 3 (complete platform matrix: desktop/iOS/Android — Codex+Fable+suite); maintainer trust + code-read approved. Rebuilt + re-verified on master carrying #5685+#5681 (all three scroll fixes coexist, ui.js valid, scope-undef+ruff clean).

• fix(webui): refuse stale-anchor scroll restore during streaming (#5637) #5666 (@allenliang2022) — residual mobile Mobile scroll jump-back: off-screen .msg-row content-visibility (#5541) collapses scrollHeight → browser clamps scrollTop #5637 fix. During streaming, refuses the stale viewport-anchor restore (which nudged a scrolled-up reader backward each tick) and lets native scroll-anchoring hold instead. Carefully platform-scoped: ONLY Android (working overflow-anchor); excludes iOS WebKit (overflow-anchor inert) and desktop (overflow-anchor:none) via a pointer:coarse matchMedia probe that resists the inline-override mutation. scrollHeightAtCapture detects content growth between capture and restore.

Gate: scope-undef CLEAN, ruff CLEAN, node -c OK, full suite (running). +522, static/ui.js + new tests/test_issue5637_stale_anchor_guard.py (395 lines). Attribution: @allenliang2022.

[greptile-apps]

Greptile Summary

This release PR introduces a streaming stale-anchor guard for Android Chrome that refuses the JS viewport-anchor restore during a live stream when the browser's native overflow-anchor layer can hold the reader's position on its own, fixing the residual ~few-hundred-px backward drift per streaming tick first reported in #5637.

• static/ui.js: Adds scrollHeightAtCapture to the anchor capture object, two new platform-detection helpers (_isIOSWebKit, _isTouchLikeMessageViewport), and two cooperative guards — one in _restoreMessageViewportAnchor and one in _restoreMessageScrollSnapshotSameFrame — that refuse stale writes when content grew since capture, no real user scroll intent is detected, and the platform is confirmed to have a working overflow-anchor engine (Android only; iOS and desktop are explicitly excluded).
• tests/test_issue5637_stale_anchor_guard.py: 395-line regression suite covering both guards, the predicate stability under inline overflowAnchor mutation, and platform-specific edge cases (iOS iPhone, iPadOS 13+ Mac-UA masquerade, Android, desktop fine-pointer).

Confidence Score: 4/5

Safe to merge; the guards are narrowly scoped to Android touch viewports and fall through to prior behavior on every other platform.

The platform detection, content-growth probe, and intent check are all individually correct and well-exercised by the test suite. The one small inconsistency — the anchor guard's Number.isFinite(_shAtCap) check lacks the > 0 guard that the sibling snapshot guard carries — is unreachable in production today (the capture code always writes a positive scrollHeight), but noting it keeps both guards in sync for future resilience.

static/ui.js — specifically the realign guard's Number.isFinite(_shAtCap) condition around line 1120.

Important Files Changed

Filename	Overview
static/ui.js	Core logic change: adds scrollHeightAtCapture to anchor snapshots, two platform-detection helpers, and two streaming stale-anchor guards. Guards are well-scoped and backed by tests; one minor defensive-coding inconsistency noted (see comment).
tests/test_issue5637_stale_anchor_guard.py	New regression suite with thorough coverage of both guards, predicate stability (inline-override mutation), iOS/iPadOS exclusion, Android positive case, and desktop regression gate. Logic is correct.
CHANGELOG.md	Release changelog entry added for #5666/#5637 — appropriate for this release PR.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Streaming tick fires\nrestore scroll snapshot] --> B{restoredViaAnchor?}
    B -- yes --> C[_restoreMessageViewportAnchor]
    B -- no --> D[_restoreMessageScrollSnapshotSameFrame\nabsolute fallback path]

    C --> E{scrollHeightAtCapture\npresent on anchor?}
    E -- no / NaN --> F[Legacy behavior:\nrun realign write]
    E -- yes --> G{content grew since\ncapture AND no intent\nAND delta > 8px?}
    G -- no --> F
    G -- yes --> H{_isTouchLikeMessageViewport?}
    H -- desktop / iOS --> F
    H -- Android touch --> I[Refuse realign\nreturn false\nbrowser overflow-anchor holds]

    D --> J{snapshot.userUnpinned===true?}
    J -- yes --> K[Skip absolute write\nearly return]
    J -- no --> L{snapshot.scrollHeight present\nAND content grew AND no intent\nAND write would move > 8px?}
    L -- no --> M[Write el.scrollTop = target]
    L -- yes --> N{_isTouchLikeMessageViewport?}
    N -- desktop / iOS --> M
    N -- Android touch --> O[Refuse absolute write\nlatch userUnpinned=true\nearly return]

    style I fill:#f96,color:#000
    style O fill:#f96,color:#000
    style K fill:#fcc,color:#000

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A[Streaming tick fires\nrestore scroll snapshot] --> B{restoredViaAnchor?}
    B -- yes --> C[_restoreMessageViewportAnchor]
    B -- no --> D[_restoreMessageScrollSnapshotSameFrame\nabsolute fallback path]

    C --> E{scrollHeightAtCapture\npresent on anchor?}
    E -- no / NaN --> F[Legacy behavior:\nrun realign write]
    E -- yes --> G{content grew since\ncapture AND no intent\nAND delta > 8px?}
    G -- no --> F
    G -- yes --> H{_isTouchLikeMessageViewport?}
    H -- desktop / iOS --> F
    H -- Android touch --> I[Refuse realign\nreturn false\nbrowser overflow-anchor holds]

    D --> J{snapshot.userUnpinned===true?}
    J -- yes --> K[Skip absolute write\nearly return]
    J -- no --> L{snapshot.scrollHeight present\nAND content grew AND no intent\nAND write would move > 8px?}
    L -- no --> M[Write el.scrollTop = target]
    L -- yes --> N{_isTouchLikeMessageViewport?}
    N -- desktop / iOS --> M
    N -- Android touch --> O[Refuse absolute write\nlatch userUnpinned=true\nearly return]

    style I fill:#f96,color:#000
    style O fill:#f96,color:#000
    style K fill:#fcc,color:#000

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[greptile-apps]

The anchor guard gates on Number.isFinite(_shAtCap) but omits the > 0 check that the sibling fallback guard uses for _snapSH. Number(null) is 0, which is finite, so an anchor with scrollHeightAtCapture explicitly set to null would enter the guard with _grewSinceCapture = (container.scrollHeight - 0) > 4 always true for any real page. The capture code always writes container.scrollHeight (a positive integer), so the path is unreachable today, but adding the parallel > 0 guard makes the two guards consistent and makes the invariant explicit.

Suggested change

	const _shAtCap=Number(anchor.scrollHeightAtCapture);
	if(Number.isFinite(_shAtCap)){
	const _shAtCap=Number(anchor.scrollHeightAtCapture);
	if(Number.isFinite(_shAtCap)&&_shAtCap>0){