fixes #14194 #6607

Description

This pull request fixes #14194 as well as keeps fixing #6607

Test cases are added to cover both #14194 & #6607

Motivation and Context

resolves #14194

How Has This Been Tested?

visually | unit

Markup:

// Paste your FULL Playground.vue here
<template>
  <v-container>
     <v-combobox
        clearable
        dense
        filled
        hide-selected
        multiple
        persistent-hint
        small-chips
        :items="['a', 'aa', 'aaa', 'bar']"
        @update:list-index="console.log($event)"
      ></v-combobox>

      <v-combobox :items="todos" :auto-select-first="true"></v-combobox>
  </v-container>
</template>

<script>
export default {
   data: () => ({
    console,
    todos: [
      { text: "Learn JavaScript", done: false },
      { text: "Learn Vue", done: false },
      { text: "Play around in JSFiddle", done: true },
      { text: "Build something awesome", done: true }
    ]
  }),
  mounted () {
  }
};
</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)