fix(VCombobox): correctly handle duplicate items

[vibhash-singh]

Description

Correctly handle duplicate item addition in combobox. Fixes #12351

Motivation and Context

Combobox removes the saved value on blur when an input string matches an existing item. This only happens with the list of objects and works fine with the list of strings. When we input a string in the combobox it is checked for the duplicate in the existing selected item list. This is done in updateTags() in VCombobox.ts

const index = this.selectedItems.indexOf(this.internalSearch)

This check will always fail for objects, as it is compared with a string. Then we try to add the search value to the list of selected items in selectItem() in VSelect.ts . There this value is found in the list of items and hence deselected, which removes it from the selected items list.

(this.internalValue || []).findIndex((i: object) => this.valueComparator(this.getValue(i), itemValue))

i !== -1 ? internalValue.splice(i, 1) : internalValue.push(item)

The fix is to correctly search and add object in the selected items list.

How Has This Been Tested?

Unit testing and Visually

Markup:

// Paste your FULL Playground.vue here
<template>
  <v-app id="inspire">
    <v-container fluid>
      <v-combobox
        v-model="model"
        :items="items"
        label="Search for an option"
        multiple
        small-chips
        @change = change
      >
      </v-combobox>
    </v-container>
  </v-app>      
</template>

<script>
export default {
 data: () => ({
    items: [
      {
        text: 'Foo',
        value: 'Foo'
      },
      {
        text: 'Bar',
        value: 'Bar'
      },
    ],
    model: [
      {
        text: 'Foo',
        value: 'Foo'
      },
    ]
  }),

  methods: {
    change() {
      console.log(this.model)
    }
  }
}
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[johnleider]

I wanted some more time to test this, will most likely go out with v2.4.12.

[johnleider]

Looks good, please resolve conflicts.

[vibhash-singh]

@johnleider Done. Please check.