[Vuejs]-Creating an expandable app bar with datepicker in vuetify

Use the extension-height prop of the app-bar..

    <v-app-bar dark extension-height="100%">
          <v-toolbar-title>Page title</v-toolbar-title>
          <v-spacer></v-spacer>
          <v-btn icon @click="extended=!extended">
              <v-icon>mdi-calendar</v-icon>
          </v-btn>
          <template v-slot:extension v-if="extended">
              <v-date-picker full-width dark />
          </template>
    </v-app-bar>

https://codeply.com/p/tJ3HM4HRZg