fix(VVirtualScroll): preserve measured heights when items change to prevent scroll jump - #23066

Description

When the items array changes (e.g. an item is removed via splice), the VVirtualScroll component completely resets its sizes and offsets arrays. This causes all previously measured heights to be lost, and offsets are recalculated using the default 16px estimate — which is far from actual measured heights — causing a visible scroll position jump.

Root Cause

In packages/vuetify/src/composables/virtual.ts, the watch(items, ...) handler:

watch(items, () => {
    sizes = Array.from({ length: items.value.length })  // All null!
    offsets = Array.from({ length: items.value.length }) // All 0!
    updateOffsets.immediate()
    calculateVisibleItems()
}, { deep: 1 })

Every item change discards all measured heights, forcing getSize() to fall back to the default itemHeight (16px). If actual items are 50px tall, offsets[500] drops from 25000 to 8000, causing paddingTop and the scroll position to jump.

Fix

Preserve existing height measurements for items that remain at the same index after the items array changes:

watch(items, (newItems, oldItems) => {
    const oldSizes = sizes.slice()
    sizes = Array.from({ length: newLength })
    offsets = Array.from({ length: newLength })

    // Preserve existing height measurements for items that remain
    // at the same index
    const minLength = Math.min(oldLength, newLength)
    for (let i = 0; i < minLength; i++) {
      if (oldSizes[i]) {
        sizes[i] = oldSizes[i]
      }
    }

    updateOffsets.immediate()
    calculateVisibleItems()
}, { deep: 1 })

This keeps offsets largely accurate when items are added or removed, preventing the scroll position from jumping.

Closes #22610