fix(VSelect): improve selection performance

[KaelWD]

Description

fixes #20703

Time to run transformIn, primitive values:

Item #	Before	After	After
1k	320ms	56ms	0.5ms
10k	40sec	780ms	1ms
100k	∞	75sec	8ms
1M	∞	∞	125ms

Object values ({ key: i }):

Item #	Before	After
1k	900ms	33ms
10k	88sec	2.5sec
100k	∞	∞
1M	∞	∞

return-object with item-value set appropriately uses primitive value lookup

Markup:

<template>
  <v-app>
    <v-container>
      <v-autocomplete
        v-model="selectedItems"
        :items="items"
        label="test"
        multiple
      >
        <template #prepend-item>
          <v-list-item @click="selectAll()">
            <v-list-item-title> Select all </v-list-item-title>
          </v-list-item>
        </template>
        <template #selection="{ item, index }">
          <template v-if="selectedItems.length > 5">
            <span v-if="!index">{{ selectedItems.length }}</span>
          </template>
          <template v-else>
            <span v-if="index" class="hint mr-1"> or </span>
            <v-chip
              :style="{ height: '24px' }"
              size="small"
              variant="flat"
              label
            >
              {{ item.title }}
            </v-chip>
          </template>
        </template>
      </v-autocomplete>
    </v-container>
  </v-app>
</template>

<script setup>
  import { ref } from 'vue'

  const selectedItems = ref([])
  const items = Array.from({ length: 10000 }, (_, i) => ({ title: `Item ${i}`, value: i }))

  function selectAll () {
    selectedItems.value =
      items.length === selectedItems.value.length ? [] : items.map(c => c.value)
  }
</script>

[johnleider]

• Is there a reason for valueComparator vs _valueComparator?
• For selectStrategies, how did you test the differences?
• It seems that most of the performance boost comes from caching the valueComparator, the other changes don't seem to do much

[KaelWD]

• There was no valueComparator variable in scope already
• There's no behaviour difference, the map is created in strategy.in in the first place so cloning it for every iteration is just a waste of performance
• Caching hasNullItem and returnObject is worth 100ms with a million items. new Map(map) takes 24ms with just 100 items. The double filter is pretty quick but might as well save the gc on an array that's going to be thrown away instantly.

[KaelWD]

There's still a O(n^2) deepEqual to find the first selected item index:

vuetify/packages/vuetify/src/components/VSelect/VSelect.tsx

Lines 317 to 319 in c4ba8bf

	const index = displayItems.value.findIndex(
	item => model.value.some(s => (props.valueComparator || deepEqual)(s.value, item.value))
	)

And a linear search to figure out if we're adding or removing a selection:

vuetify/packages/vuetify/src/components/VSelect/VSelect.tsx

Line 266 in c4ba8bf

const index = model.value.findIndex(selection => (props.valueComparator || deepEqual)(selection.value, item.value))