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… element shrinks With `anchorTo: 'end'`, resizeItem keeps the end in view when an item grows, but nothing did the same when the scroll element itself shrinks (the window resizing, or the app changing the element's height). The browser keeps `scrollTop`, so the last items dropped below the fold. Rows sized from the viewport (e.g. capped with `vh`) made it worse: they shrink in the same frame, and were then compensated as rows above a viewport no longer at the end. The scroll element's rect callback now applies the same rule as resizeItem: if the viewport was pinned to the end against its previous size and the element shrank along the scroll axis, scroll by the shrink. Like resizeItem it keeps any distance from the end that was within `scrollEndThreshold`, and it never scrolls past the element's real end, so a border on the element or content shorter than the viewport cannot overshoot. Growth and cross-axis changes are left alone.
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Included review availability: This review used your included allowance. Your plan provides up to 8 included reviews per hour; 7 remain after this review. 📝 WalkthroughWalkthroughThe virtualizer now adjusts end-anchored scrolling when the scroll element shrinks and the viewport was near the end. Unit tests and chat end-to-end tests cover viewport resizing. Documentation describes the behavior. ChangesEnd-anchor viewport resize
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Bug fix · Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant ResizeObserver
participant Virtualizer
participant ScrollElement
ResizeObserver->>Virtualizer: report smaller scroll rectangle
Virtualizer->>Virtualizer: check end threshold and smooth-scroll state
Virtualizer->>ScrollElement: apply scroll adjustment
Suggested reviewers: Merge Risk: ⚪ Minimal · up to The viewport-shrink change has no identified issue that needs resolution before merge. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Actionable comments posted: 1
- 🪄 Fix CodeRabbit comments on this PR
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Review comments at @packages/virtual-core/src/index.ts:
- Line 907: In the rectangle callback’s shrink handling, use
getVirtualDistanceFromEnd() to cap the shrink delta when _clampedAdjustment is
pending; otherwise retain getDistanceFromEnd(). Add a test that commits the
grown sizer after both callbacks and verifies the viewport reaches the end.
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.changeset/end-anchor-scroll-element-resize.mddocs/api/virtualizer.mddocs/chat.mdpackages/react-virtual/e2e/app/chat/main.tsxpackages/react-virtual/e2e/app/test/chat.spec.tspackages/virtual-core/src/index.tspackages/virtual-core/tests/index.test.ts
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…s pending If an end-pinned item grew before the sizer did, its compensation write is clamped and waits to be retried once the sizer grows. A scroll element shrink in that window was capped at the DOM distance to the end, which is stale until the sizer commits, so the shrink was dropped and the retry landed short of the new end. Apply the whole shrink while a clamped write is pending; the retry lands it.
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Thanks, and I agree it belongs in core.
Added two comments before merge, also let's extract the pinned-to-end to helper
// Whether an end-anchored viewport should follow size changes to stay at
// the end. Shared by item growth (`resizeItem`) and scroll element shrink.
private isPinnedToEnd = () => {
return (
this.options.anchorTo === 'end' &&
this.scrollState?.behavior !== 'smooth' &&
this.getVirtualDistanceFromEnd() <= this.options.scrollEndThreshold
)
}| // browser keeps scrollTop, so the end would otherwise drop below the | ||
| // fold. Judge "pinned" against the size before this change, and | ||
| // never scroll past the element's real end. | ||
| const prevSize = this.scrollRect !== null ? this.getSize() : null |
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Swapping the scroll element carries the pin over to the new element, cleanup() doesn't reset scrollRect or scrollOffset, and observeElementRect reports the new element's size right away, before observeElementOffset is set up.
So when getScrollElement() returns a new, shorter element, this compares the old element's size and offset, and writes scrollTo(300, { adjustments: 50 }) to the new one.
The first report for each element has nothing to compare against, so I'd skip it:
// The first report for a new element has nothing to compare against:
// `scrollRect` and `scrollOffset` still describe the previous element.
let isFirstRect = true
this.unsubs.push(
this.options.observeElementRect(this, (rect) => {
const prevSize =
!isFirstRect && this.scrollRect !== null ? this.getSize() : null
isFirstRect = false
// ...| const wasAtEnd = | ||
| prevSize !== null && | ||
| this.options.anchorTo === 'end' && | ||
| this.scrollState?.behavior !== 'smooth' && | ||
| this.getVirtualDistanceFromEnd() <= this.options.scrollEndThreshold |
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A reader scrolling up gets pulled back to the end.
If the viewport shrinks while the user is scrolling up but still within scrollEndThreshold, this scrolls them back down. With a window scroller on iOS that happens on every scroll-up gesture near the end: the URL bar reappears, innerHeight shrinks, and the adjustment is deferred and applied after touch-end, pulling them down by the bar's height.
Scrolling up means they're leaving the end, so I'd skip it:
const wasAtEnd =
prevSize !== null &&
this.scrollDirection !== 'backward' &&
this.isPinnedToEnd()
I only added this to the rect path and left resizeItem as it is. Let me know if you see a case where this is wrong, or if you think resizeItem should get the same guard for consistency. If you keep it, the docs line could note that scrolling up opts out.
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2 participants
Fixes #1297
With anchorTo: 'end', a list pinned to its newest item loses the end when the scroll element gets shorter, for example on a window resize or when the app changes its height. The last items end up below the fold.
🎯 Changes
Root cause.
resizeItemkeeps an end-pinned viewport at the end when an item grows, but the scroll element's rect callback only records the new size. The browser keepsscrollTop, so the end drops below the fold by the amount the element shrank.Fix. Apply
resizeItem's rule in the rect callback: if the viewport was withinscrollEndThresholdof the end against its previous size and the element shrank along the scroll axis, scroll by the shrink. Like resizeItem, it moves by the amount the element shrank, so a reader slightly above the end stays the same distance above it. It never scrolls further than the element can actually scroll, so a border on the element, or content shorter than the viewport, can't push it past the end. Growth and cross-axis changes are left to the browser.Tests.
chate2e: shrinking the scroll container, and shrinking it together with a row above the viewport. Both fail onmain.Tests.
Core: a shrink while pinned keeps the end; a grow, a width-only change and a shrink while reading history leave the offset alone; a reader within the threshold keeps their gap; an item above the viewport shrinking in the same frame keeps the end with either callback order.
react-virtual chat e2e: shrinking the scroll container, and shrinking it together with a row above the viewport. Both fail on main.
✅ Checklist
pnpm run test:pr.🚀 Release Impact
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