fix(VVirtualScroll): preserve height measurements when items array changes

[waterWang]

Description

When using v-virtual-scroll with item-height="null" (dynamic height), the sizes array was completely reset whenever the items array was reassigned, causing all previously measured heights to be lost. This caused the scroll position to jump unexpectedly when:

1. Expanding/collapsing rows in a subgrid
2. Updating data that causes the items array to be reassigned
3. Any change that triggers items reactivity

Root Cause

In packages/vuetify/src/composables/virtual.ts, the watch(items, ...) handler at line 294 was resetting both sizes and offsets arrays to empty arrays of the new length, discarding all previously measured heights.

Fix

Instead of creating empty arrays, the fix preserves existing height measurements for items at the same index:

• Save the old sizes array before resetting
• After creating new arrays of the correct length, restore heights for items at matching indices
• Items at new indices (beyond the old length) remain unmeasured (will use estimated heights)
• Items at removed indices (beyond the new length) are simply dropped

This way, scroll position remains stable when the items array changes but contains similar items.

Related Issue

Fixes #22590

[J-Sek]

duplicate of #23066