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fix(lit-virtual): add setOptions to VirtualizerController for reactive option updates #1253

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fix(lit-virtual): add setOptions to VirtualizerController for reactive option updates #1253

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5ccde70
fix(lit-virtual): add setOptions to VirtualizerController for reactiv…
waterWang Aug 12, 2026
e548ded
test(lit-virtual): add setOptions test for reactive option updates
waterWang Aug 12, 2026
fe62dbe
ci: apply automated fixes
autofix-ci[bot] Aug 18, 2026
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18 changes: 18 additions & 0 deletions packages/lit-virtual/src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -18,12 +18,15 @@ class VirtualizerControllerBase<

private readonly virtualizer: Virtualizer<TScrollElement, TItemElement>

private options: VirtualizerOptions<TScrollElement, TItemElement>

private cleanup: () => void = () => {}

constructor(
host: ReactiveControllerHost,
options: VirtualizerOptions<TScrollElement, TItemElement>,
) {
this.options = options
const resolvedOptions: VirtualizerOptions<TScrollElement, TItemElement> = {
...options,
onChange: (instance, sync) => {
Expand All @@ -39,6 +42,21 @@ class VirtualizerControllerBase<
return this.virtualizer
}

public setOptions(
options: Partial<VirtualizerOptions<TScrollElement, TItemElement>>,
) {
this.options = { ...this.options, ...options }

@piecyk piecyk Aug 18, 2026

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A plain spread lets an explicit undefined wipe a stored option: setOptions({ estimateSize: undefined }) leaves estimateSize undefined for good, and core then falls back to its own default rather than the value set earlier.

Core's setOptions guards against exactly this (it skips keys whose value is undefined) — worth matching here.

const resolvedOptions: VirtualizerOptions<TScrollElement, TItemElement> = {

@piecyk piecyk Aug 18, 2026

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This resolvedOptions block is a copy of the one in the constructor, and the two close over different things (options vs this.options). Harmless today, but a shared private resolveOptions(options) used by both would stop them drifting.

...this.options,
onChange: (instance, sync) => {
this.host.updateComplete.then(() => this.host.requestUpdate())
this.options.onChange?.(instance, sync)
},
}
this.virtualizer.setOptions(resolvedOptions)
this.virtualizer._willUpdate()
}

hostConnected() {
this.cleanup = this.virtualizer._didMount()
}
Expand Down
83 changes: 83 additions & 0 deletions packages/lit-virtual/tests/index.test.ts
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Expand Up @@ -113,3 +113,86 @@ test('should render virtual items', async () => {
'Element did not render virtual items',
)
})

test('should apply updated options via setOptions', async () => {
@customElement('test-list-setoptions' as any)
class ListSetOptions extends LitElement {
private scrollElementRef: Ref<HTMLDivElement> = createRef()

private virtualizerController: VirtualizerController<
HTMLDivElement,
Element
>

constructor() {
super()
this.virtualizerController = new VirtualizerController(this, {
getScrollElement: () => this.scrollElementRef.value,
count: 10,
estimateSize: () => 50,
observeElementRect: (_, cb) => {
cb({ height, width })
},
})
}

render() {
this.virtualizerController.setOptions({

@piecyk piecyk Aug 18, 2026

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Calling setOptions inside render() means the count is 20 from the very first paint, so nothing is actually updated reactively — the test would pass without any reactivity at all.

Driving it from a reactive @property in willUpdate is what the feature is for, and it avoids modelling a pattern that mutates controller state during render.

count: 20,
})
const virtualizer = this.virtualizerController.getVirtualizer()
const virtualRows = virtualizer.getVirtualItems()
return html`
<div class="list scroll-container" ${ref(this.scrollElementRef)}>
<div
style="position: relative; height: ${virtualizer.getTotalSize()}px; width: 100%;"
>
<div
style="position:absolute;top:0;left:0;width:100%;transform:translateY(${virtualRows[0]
? virtualRows[0].start
: 0}px);"
>
${repeat(
virtualRows,
(virtualRow: any) => virtualRow.key,
(virtualRow: any) =>
html` <div
data-index="${virtualRow.index}"
class="${virtualRow.index % 2 === 0
? 'list-item-even'
: 'list-item-odd'}"
>
<div style="padding: 10px 0;">
<div>Row ${virtualRow.index}</div>
<div>Item ${virtualRow.index}</div>
</div>
</div>`,
)}
</div>
</div>
</div>
<style>
.list {
border: 1px solid #e6e4dc;
max-width: 100%;
}
.scroll-container {
height: ${height}px;
width: ${width}px;
overflow-y: auto;
}
</style>
`
}
}

const el = await fixture(
html`<test-list-setoptions></test-list-setoptions>` as any,
)
await elementUpdated(el)
await waitUntil(
() => el.shadowRoot.querySelector('[data-index="15"]'),

@piecyk piecyk Aug 18, 2026

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This is what fails. Index 15 is never rendered: the viewport is 400px with estimateSize: () => 50, so only about indexes 0–9 render at scroll offset 0 — with count: 10 and with count: 20 alike. Raising the count doesn't widen the rendered window, so the test would fail even if setOptions were perfect.

getTotalSize() is what actually moves (500 → 1000). Assert that instead, and assert the initial 500 too, so the test also covers the constructor options — right now nothing checks the starting count of 10 took effect.

The follow-up expect(...).toBeTruthy() on line 197 is also redundant; waitUntil already guarantees it.

'Element did not render items beyond initial count of 10',
)
expect(el.shadowRoot.querySelector('[data-index="15"]')).toBeTruthy()
})