fix(VDataTable): show correct items count after expanding group

[huynhtehoa]

Description

fixes #20313

• Problem: items slot prop contains duplicated items when a group is expanded

• Cause: items slot prop in the parent component is paginatedItemsWithoutGroups. This items are first flattened from its group in useGroupedItems (VDataTable.jsx:163) and then extracted to rows in extractRows (VDataTable.jsx:169). The extraction again flattens the items from its group which causes the duplication

• Solution: make extractRows operation returns unique items

Markup:

I reused reproduction provided by #20313

<template>
  <v-data-table
    :group-by="groupBy"
    :headers="headers"
    :items="desserts"
    item-value="name"
    items-per-page="-1"
  >
    <template #body.append="{ items }"> {{ items.length }} </template>
  </v-data-table>
</template>

<script>
  export default {
    data() {
      return {
        groupBy: [
          {
            key: 'gluten',
            order: 'asc',
          },
        ],
        headers: [
          {
            title: 'Dessert (100g serving)',
            align: 'start',
            sortable: false,
            key: 'name',
          },
          { title: 'Calories', key: 'calories' },
          { title: 'Fat (g)', key: 'fat' },
          { title: 'Carbs (g)', key: 'carbs' },
          { title: 'Protein (g)', key: 'protein' },
          { title: 'Iron (%)', key: 'iron' },
        ],
        desserts: [
          {
            name: 'Frozen Yogurt',
            calories: 159,
            fat: 6.0,
            carbs: 24,
            protein: 4.0,
            iron: '1%',
            gluten: false,
          },
          {
            name: 'Ice cream sandwich',
            calories: 237,
            fat: 9.0,
            carbs: 37,
            protein: 4.3,
            iron: '1%',
            gluten: false,
          },
          {
            name: 'Eclair',
            calories: 262,
            fat: 16.0,
            carbs: 23,
            protein: 6.0,
            iron: '7%',
            gluten: true,
          },
          {
            name: 'Cupcake',
            calories: 305,
            fat: 3.7,
            carbs: 67,
            protein: 4.3,
            iron: '8%',
            gluten: true,
          },
          {
            name: 'Gingerbread',
            calories: 356,
            fat: 16.0,
            carbs: 49,
            protein: 3.9,
            iron: '16%',
            gluten: true,
          },
          {
            name: 'Jelly bean',
            calories: 375,
            fat: 0.0,
            carbs: 94,
            protein: 0.0,
            iron: '0%',
            gluten: false,
          },
          {
            name: 'Lollipop',
            calories: 392,
            fat: 0.2,
            carbs: 98,
            protein: 0,
            iron: '2%',
            gluten: false,
          },
          {
            name: 'Honeycomb',
            calories: 408,
            fat: 3.2,
            carbs: 87,
            protein: 6.5,
            iron: '45%',
            gluten: true,
          },
          {
            name: 'Donut',
            calories: 452,
            fat: 25.0,
            carbs: 51,
            protein: 4.9,
            iron: '22%',
            gluten: true,
          },
          {
            name: 'KitKat',
            calories: 518,
            fat: 26.0,
            carbs: 65,
            protein: 7,
            iron: '6%',
            gluten: true,
          },
        ],
      }
    },
  }
</script>

[johnleider]

I researched your fix and think it's a very clever way to dedupe the array. Bravo