measureElement caches 0 size for nodes that aren't attached yet (Vue: rows stack at start 0 and the list scrolls away on mount)

[webn00b]

Describe the bug

Virtualizer.measureElement(node) measures the node synchronously right after observing it, even when the node is not attached to the document yet. A detached node reports offsetHeight === 0, and since there is no cached size on the first measurement, that 0 is stored as the item size.

In Vue this is easy to hit. Function refs run synchronously during patch, so when the rows are created in the same patch as their parent (the sizer is behind v-if and shows up once data is loaded), :ref="(el) => virtualizer.measureElement(el)" is called while the sizer is still detached:

1. Every visible row gets size 0 and renders at start: 0, on top of each other.
2. On the next frame the ResizeObserver reports the real sizes and the rows spread out.
3. The list ends up scrolled down (~305px in the repro, row 9 at the top) instead of staying at the top.

The ResizeObserver callback already ignores disconnected nodes (#1135), but the sync path in measureElement doesn't. #1226 only rejects a 0 when there is a previously cached size, so it doesn't cover the first measurement either.

Your minimal, reproducible example

https://stackblitz.com/github/webn00b/tanstack-vue-virtual-detached-measure

Steps to reproduce

1. Open the repro. The scroll container is always rendered; the sizer and the rows are behind v-if="loaded", which flips after a simulated fetch.
2. Rows are measured with :ref="(el) => virtualizer.measureElement(el)", estimateSize is 60.
3. Check the output under the list: the ref calls log isConnected: false, offsetHeight: 0, and right after nextTick the visible items have start: 0, size: 0.
4. After the ResizeObserver fires, the list is at scrollTop ≈ 305.

Expected behavior

Rows keep their estimateSize positions until they are measured, and the list stays at scrollTop: 0.

How often does this bug happen?

Every time

Screenshots or Videos

No response

Platform

• OS: macOS
• Browser: Chrome

Not browser-specific, it comes from when Vue calls function refs.

tanstack-virtual version

@tanstack/vue-virtual 3.13.39, @tanstack/virtual-core 3.17.11

TypeScript version

No response

Additional context

Skipping the sync measurement when !node.isConnected fixes it. The node is still observed, so the first frame uses estimateSize, the ResizeObserver delivers the real size once the node is attached, and the scroll position stays at 0. I have a fix with a regression test and can open a PR.

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