feat(virtual-core): iOS momentum-safe scroll adjustments via CSS offset#1189
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📝 WalkthroughWalkthroughDefers visual scroll adjustments on iOS WebKit into a CSS margin offset during touch/momentum, folds the deferred delta into offset reads, flushes the compensation when safe or before programmatic scrolls, refines backward-scroll resize adjustment based on measurement cache, and updates tests, docs, example CSS, and a changeset. ChangesiOS Momentum Scroll Safety and Adjustment Refinement
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🧹 Nitpick comments (1)
packages/virtual-core/src/index.ts (1)
742-756: 💤 Low valueConsider edge case:
scrollOffsetis null when force-flushing.In
_forceFlushIosDeviation, ifscrollOffsetisnulland_iosDeferredAdjustmentis non-zero,rawBrowserScrollwould compute as a negative value (0 - adjustment). While this scenario is unlikely in practice (deviation is only accumulated after scroll events setscrollOffset), a defensive guard would prevent unexpected behavior if the invariant is violated.🛡️ Optional defensive guard
private _forceFlushIosDeviation = () => { if (this._iosDeferredAdjustment === 0) return + if (this.scrollOffset === null) { + // No scroll offset yet; just clear the deviation state + this._iosDeferredAdjustment = 0 + this._applyIosDeviation() + return + } const rawBrowserScroll = (this.scrollOffset ?? 0) - this._iosDeferredAdjustment🤖 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 742 - 756, If this._iosDeferredAdjustment is non-zero but this.scrollOffset is null, computing rawBrowserScroll yields an incorrect value; add a defensive guard at the top of _forceFlushIosDeviation to handle null scrollOffset (e.g. if this.scrollOffset == null then clear _iosDeferredAdjustment and call _applyIosDeviation and return) so you don't call _scrollToOffset with an invalid rawBrowserScroll; reference the method name _forceFlushIosDeviation and the fields scrollOffset, _iosDeferredAdjustment and the helpers _applyIosDeviation and _scrollToOffset when making the change.
🤖 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.
Nitpick comments:
In `@packages/virtual-core/src/index.ts`:
- Around line 742-756: If this._iosDeferredAdjustment is non-zero but
this.scrollOffset is null, computing rawBrowserScroll yields an incorrect value;
add a defensive guard at the top of _forceFlushIosDeviation to handle null
scrollOffset (e.g. if this.scrollOffset == null then clear
_iosDeferredAdjustment and call _applyIosDeviation and return) so you don't call
_scrollToOffset with an invalid rawBrowserScroll; reference the method name
_forceFlushIosDeviation and the fields scrollOffset, _iosDeferredAdjustment and
the helpers _applyIosDeviation and _scrollToOffset when making the change.
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.changeset/ios-momentum-scroll.mddocs/chat.mdexamples/react/chat/src/index.csspackages/virtual-core/src/index.tspackages/virtual-core/tests/index.test.ts
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🤖 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 `@docs/chat.md`:
- Line 58: Update the rationale to explain that useCallback is applied to
getItemKey to stabilize its function reference so memoized values that include
getItemKey (such as getMeasurements which depends on
this.getMeasurementOptions()) don't treat measurement options as changed on
every render; specifically mention wrapping getItemKey in useCallback to keep a
stable reference, how passing a new getItemKey identity each render can
invalidate getMeasurements/getMeasurementOptions memoization and cause
unnecessary measurement rebuilds/cache invalidation, and note this keeps the
virtualizer from triggering extra recomputation while clarifying that edge-key
change detection itself is based on returned key values.
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.changeset/ios-momentum-scroll.mddocs/chat.mdexamples/react/chat/src/index.csspackages/virtual-core/src/index.tspackages/virtual-core/tests/index.test.ts
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- packages/virtual-core/src/index.ts
- packages/virtual-core/tests/index.test.ts
On iOS WebKit, writing scrollTop during momentum scroll cancels the in-flight scroll. Instead of writing scrollTop, apply a negative marginTop on the container element to visually compensate for above-viewport size changes. The CSS offset is flushed to a real scrollTop write once momentum fully settles. Changes: - Add CSS offset (marginTop) approach for iOS scroll adjustments - Defer adjustments through touch→momentum→settled lifecycle - Force-flush CSS offset before programmatic scroll operations - Compensate scrollOffset for CSS offset in range calculations - Guard against Safari elastic overscroll during flush - Clean up CSS offset on unmount - Refine backward-scroll suppression: first measurements always adjust (needed for prepend), re-measurements skip during backward scroll (avoids scrollTop cascade jank) - Add overflow-anchor: none to chat example CSS - Update chat docs with iOS section and getItemKey best practices
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On iOS WebKit, writing
scrollTopduring momentum scroll cancels the in-flight scroll. Instead of writingscrollTop, we now apply a negativemarginTopon the container element to visually compensate for above-viewport size changes. The CSS offset is flushed to a realscrollTopwrite once momentum fully settles.marginTop/marginLeft)scrollToIndex,scrollToOffset,scrollBy)scrollOffsetfor active CSS offset in range calculationsscrollTopcascade jank🎯 Changes
✅ Checklist
pnpm run test:pr.🚀 Release Impact
Summary by CodeRabbit
Improvements
Behavior
Documentation
Tests