fix(virtual-core): eagerly adjust scrollOffset on prepend to prevent jump#1176
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tannerlinsley merged 1 commit intoJun 1, 2026
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📝 WalkthroughWalkthroughEagerly resolve append/prepend scroll anchors inside setOptions (precompute anchorDelta and update scrollOffset), record pendingScrollAnchor, and have _willUpdate consume the precomputed delta to sync DOM (with iOS deferral). A changeset and a test expectation were added/updated. ChangesScroll Anchor Lifecycle Optimization
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Actionable comments posted: 1
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
Inline comments:
In `@packages/virtual-core/src/index.ts`:
- Around line 823-834: Tests expect numeric options.adjustments but
_scrollToOffset now forwards undefined for the _willUpdate anchor sync path;
update the two assertions in packages/virtual-core/tests/index.test.ts that
reference anchorTo:end to expect undefined for options.adjustments instead of
100 and 70 (specifically the tests "anchorTo:end keeps visible content stable
when older items are prepended" and "anchorTo:end keeps a pinned streaming
message pinned as it grows"); leave the iOS deferred-flush test that asserts 50
unchanged because it exercises the iOS accumulator path, and ensure references
to _scrollToOffset and scrollToFn are used when reviewing the behavior change.
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🧹 Nitpick comments (1)
packages/virtual-core/tests/index.test.ts (1)
2267-2280: ⚡ Quick winAssert the eager correction before
_willUpdate().This only proves the later DOM sync. The regression here was the one-frame wrong range during render, so it would be better to assert
scrollOffsetorgetVirtualItems()immediately aftersetOptions()and before_willUpdate()runs.🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the rest with a brief reason, keep changes minimal, and validate. In `@packages/virtual-core/tests/index.test.ts` around lines 2267 - 2280, The test currently only checks the post-DOM-sync scrollTo call; add an assertion that verifies the eager correction happened synchronously before the internal update by checking the virtualizer's scrollOffset or getVirtualItems() immediately after calling setMessages (and before _willUpdate() runs). Specifically, after calling setMessages([{ id: 'm--2' }, { id: 'm--1' }, ...messages]) assert virtualizer.scrollOffset (or virtualizer.getVirtualItems()) has the expected value/range to prove the one-frame correction occurred, and keep the existing scrollToFn assertions for the later DOM sync; locate this in the same test around the setMessages call and place the new assertion before any code that triggers or waits for _willUpdate().
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
Inline comments:
In `@packages/virtual-core/src/index.ts`:
- Around line 830-838: The direct call to _scrollToOffset bypasses the iOS
deferral path and can be cancelled by a prepend-triggered anchor sync; instead,
gate this DOM sync through the same iOS deferral used elsewhere or use the
existing applyScrollAdjustment path. Replace the direct
this._scrollToOffset(this.getScrollOffset(), ...) with the code path that honors
_iosDeferredAdjustment (i.e., call applyScrollAdjustment / the helper that
defers/queues adjustments when _iosDeferredAdjustment is set) so the scroll
write is deferred during iOS touch/momentum; ensure you still pass the same
offset from getScrollOffset() and preserve the adjustments/behavior options when
delegating.
---
Nitpick comments:
In `@packages/virtual-core/tests/index.test.ts`:
- Around line 2267-2280: The test currently only checks the post-DOM-sync
scrollTo call; add an assertion that verifies the eager correction happened
synchronously before the internal update by checking the virtualizer's
scrollOffset or getVirtualItems() immediately after calling setMessages (and
before _willUpdate() runs). Specifically, after calling setMessages([{ id:
'm--2' }, { id: 'm--1' }, ...messages]) assert virtualizer.scrollOffset (or
virtualizer.getVirtualItems()) has the expected value/range to prove the
one-frame correction occurred, and keep the existing scrollToFn assertions for
the later DOM sync; locate this in the same test around the setMessages call and
place the new assertion before any code that triggers or waits for
_willUpdate().
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Actionable comments posted: 1
♻️ Duplicate comments (1)
packages/virtual-core/src/index.ts (1)
830-838:⚠️ Potential issue | 🟠 Major | 🏗️ Heavy liftPreserve the iOS deferred-scroll path for this DOM sync.
This direct
_scrollToOffsetwrite bypassesapplyScrollAdjustment'sisIOSWebKit()gate. If a prepend lands during touch or momentum scrolling, this path still writesscrollTopimmediately and cancels the native scroll on iOS, even though the logicalscrollOffsetwas already fixed eagerly.🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the rest with a brief reason, keep changes minimal, and validate. In `@packages/virtual-core/src/index.ts` around lines 830 - 838, The direct call to this._scrollToOffset bypasses applyScrollAdjustment's isIOSWebKit() logic and can cancel native iOS momentum; instead of calling _scrollToOffset directly when key !== null && !followOnAppend, route the synchronization through applyScrollAdjustment (or its equivalent helper) so the iOS-specific deferred-scroll gate is respected; use this.getScrollOffset() as the target offset and pass through the adjustments/behavior params (or undefined) to applyScrollAdjustment so iOS WebKit will defer the DOM write while other platforms still sync immediately.
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
Inline comments:
In `@packages/virtual-core/src/index.ts`:
- Around line 604-615: The code may read stale measurement data when computing
newOffset via newMeasurements[idx].start; before using newMeasurements[idx]
ensure measurements are rebuilt for the current key ordering: either call your
measurement invalidation/rebuild helper (e.g., this.invalidateMeasurements() or
this.rebuildMeasurements()) or include the item key identity in the measurement
memo so getMeasurements() returns a fresh array for the current getItemKey
order; specifically, after computing idx using getItemKey and before accessing
newMeasurements[idx].start, force a measurements refresh (or verify
newMeasurements[idx] corresponds to getItemKey(idx) and rebuild if it does not)
so this.scrollOffset is computed from up-to-date measurements.
---
Duplicate comments:
In `@packages/virtual-core/src/index.ts`:
- Around line 830-838: The direct call to this._scrollToOffset bypasses
applyScrollAdjustment's isIOSWebKit() logic and can cancel native iOS momentum;
instead of calling _scrollToOffset directly when key !== null &&
!followOnAppend, route the synchronization through applyScrollAdjustment (or its
equivalent helper) so the iOS-specific deferred-scroll gate is respected; use
this.getScrollOffset() as the target offset and pass through the
adjustments/behavior params (or undefined) to applyScrollAdjustment so iOS
WebKit will defer the DOM write while other platforms still sync immediately.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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…jump When items are prepended with anchorTo: 'end' and dynamic sizes, the virtualizer would compute the wrong visible range for one frame (using stale estimate-based positions) and then correct in the next frame via _willUpdate, producing a visible "jump". Fix by eagerly adjusting scrollOffset in setOptions during the render pass so calculateRange/getVirtualItems return the correct items immediately. _willUpdate then only syncs the browser's actual scroll position to match.
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When items are prepended with anchorTo: 'end' and dynamic sizes, the virtualizer would compute the wrong visible range for one frame (using stale estimate-based positions) and then correct in the next frame via _willUpdate, producing a visible "jump".
Fix by eagerly adjusting scrollOffset in setOptions during the render pass so calculateRange/getVirtualItems return the correct items immediately. _willUpdate then only syncs the browser's actual scroll position to match.
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