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fix: avoid virtualizer losing scroll position on large size change #11256

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tomivirkki merged 2 commits into from
Mar 6, 2026
Merged

fix: avoid virtualizer losing scroll position on large size change #11256

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3691611
fix: avoid virtualizer losing scroll position on large size change
tomivirkki Mar 5, 2026
922b72b
Refactor the fix
tomivirkki Mar 6, 2026
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23 changes: 19 additions & 4 deletions packages/component-base/src/virtualizer-iron-list-adapter.js
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Original file line number Diff line number Diff line change
Expand Up @@ -333,6 +333,19 @@ export class IronListAdapter {
return element ? this.scrollTarget.getBoundingClientRect().top - element.getBoundingClientRect().top : undefined;
}

/**
* Adjusts the scroll position to compensate for any offset change of a given index.
* @param {number} index - The index whose scroll offset to restore
* @param {number|undefined} offsetBefore - The scroll offset of the index before the change
* @private
*/
__restoreScrollOffset(index, offsetBefore) {
const offsetAfter = this.__getIndexScrollOffset(index);
if (offsetBefore !== undefined && offsetAfter !== undefined) {
this._scrollTop += offsetBefore - offsetAfter;
}
}

get size() {
return this.__size;
}
Expand Down Expand Up @@ -382,10 +395,7 @@ export class IronListAdapter {
// and remove exceeding items when size is decreased.
this.scrollToIndex(fvi);

const fviOffsetAfter = this.__getIndexScrollOffset(fvi);
if (fviOffsetBefore !== undefined && fviOffsetAfter !== undefined) {
this._scrollTop += fviOffsetBefore - fviOffsetAfter;
}
this.__restoreScrollOffset(fvi, fviOffsetBefore);
}

this.__preventElementUpdates = false;
Expand Down Expand Up @@ -851,6 +861,9 @@ export class IronListAdapter {
const threshold = OFFSET_ADJUST_MIN_THRESHOLD;
const maxShift = 100;

const fvi = this.adjustedFirstVisibleIndex;
const fviOffsetBefore = this.__getIndexScrollOffset(fvi);

// Near start
if (this._scrollTop === 0) {
this._vidxOffset = 0;
Expand All @@ -872,6 +885,8 @@ export class IronListAdapter {
this._vidxOffset += Math.min(maxOffset - this._vidxOffset, maxShift);
super.scrollToIndex(this.firstVisibleIndex - (this._vidxOffset - oldOffset));
}

this.__restoreScrollOffset(fvi, fviOffsetBefore);
}
}
}
Expand Down
31 changes: 27 additions & 4 deletions packages/component-base/test/virtualizer-unlimited-size.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,11 @@ describe('unlimited size', () => {
return [...elementsContainer.children].reduce((max, el) => Math.max(max, el.index), 0);
}

function getIndexScrollOffset(fvi) {
const element = elementsContainer.querySelector(`#item-${fvi}`);
return scrollTarget.getBoundingClientRect().top - element.getBoundingClientRect().top;
}

it('should scroll to a large index', () => {
const index = Math.floor(virtualizer.size / 2);
virtualizer.scrollToIndex(index);
Expand Down Expand Up @@ -159,10 +164,18 @@ describe('unlimited size', () => {
smallestIndex = Math.min(...Array.from(elementsContainer.children).map((el) => el.index));

scrollTarget.scrollTop -= (elementHeight * elementCount) / 2;

// Scroll offset of the index before the scroll event
const scrollOffsetBefore = getIndexScrollOffset(smallestIndex);
await oneEvent(scrollTarget, 'scroll');

const item = elementsContainer.querySelector(`#item-${smallestIndex}`);
expect(item).to.be.ok;

// Scroll offset of the same index after the scroll event
const scrollOffsetAfter = getIndexScrollOffset(smallestIndex);
// Expect the scroll offset to be approximately the same, meaning the scroll position is preserved relative to the item
expect(scrollOffsetAfter).to.be.closeTo(scrollOffsetBefore, 1);
}

const item = elementsContainer.querySelector('#item-0');
Expand Down Expand Up @@ -267,8 +280,7 @@ describe('unlimited size', () => {
expect(item.getBoundingClientRect().top).to.be.closeTo(scrollTarget.getBoundingClientRect().top - 10, 1);
});

// FIXME: Fails due to a scroll offset reset caused by _adjustVirtualIndexOffset on scroll event.
it.skip('should preserve scroll position when size decrease does not affect any rendered indexes', async () => {
it('should preserve scroll position when size decrease does not affect any rendered indexes', async () => {
// Scroll to an index and add an additional scroll offset.
const index = virtualizer.size - 2000;
virtualizer.scrollToIndex(index);
Expand All @@ -282,8 +294,7 @@ describe('unlimited size', () => {
expect(item.getBoundingClientRect().top).to.be.closeTo(scrollTarget.getBoundingClientRect().top - 10, 1);
});

// FIXME: Fails due to a scroll offset reset caused by _adjustVirtualIndexOffset on scroll event.
it.skip('should preserve scroll position on size increase', async () => {
it('should preserve scroll position on size increase', async () => {
const index = virtualizer.size - 2000;
virtualizer.scrollToIndex(index);
scrollTarget.scrollTop += 10;
Expand All @@ -294,4 +305,16 @@ describe('unlimited size', () => {
const item = elementsContainer.querySelector(`#item-${index}`);
expect(item.getBoundingClientRect().top).to.be.closeTo(scrollTarget.getBoundingClientRect().top - 10, 1);
});

it('should preserve scroll position on large size decrease', async () => {
const index = 300000;
virtualizer.scrollToIndex(index);
scrollTarget.scrollTop += 10;

virtualizer.size = 500000;
await oneEvent(scrollTarget, 'scroll');

const item = elementsContainer.querySelector(`#item-${index}`);
expect(item.getBoundingClientRect().top).to.be.closeTo(scrollTarget.getBoundingClientRect().top - 10, 1);
});
});
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