fix(VAppBar): fix inability to scroll to the bottom

[lzl0304]

Description

fixes #19090

After altering scrollHeight, determining scroll direction becomes invalid.

Markup:

<template>
  <v-layout ref="app" class="rounded rounded-md">
    <v-app-bar color="grey-lighten-2" name="app-bar" scroll-behavior="hide" />
    <v-navigation-drawer
      color="grey-darken-2"
      name="drawer"
      expand-on-hover
      permanent
      rail
    />

    <v-main
      class="d-flex flex-column align-center justify-center"
      style="min-height: 300px"
    >
      <div v-for="n in 17" :key="n" class="pa-16 ma-2 w-50 bg-green text-center">
        box {{ n }}
      </div>
    </v-main>

    <v-footer class="justify-center" color="grey" name="footer" app>
      FOOTER
    </v-footer>
  </v-layout>
</template>