Viewport change of Vue3 app in Cypress gives TypeError ref.value is null

In Cypress I am changing the viewport of my Veu3 app after I preform any other test and it gives the error: "(uncaught execption) TypeError a.value is null".

The minimum app that produces the error:

<script setup>
  import { ref, onMounted } from 'vue'
  const a = ref(null)
  
  onMounted (() => {
    function onWindowResize() {
      window.addEventListener("resize", () => {
        //a.value always needs to refresh when the window resizes to different content
        a.value.textContent = 'Text displayed here will be decided on the window size'
      })
    }
    onWindowResize()
  })
</script>

<template>
<div>
  <span ref="a" data-testid="test1">M</span>
  <a> ..more</a>
</div>
</template>

With the following test. The interesting thing to note here is that if you don't include the first test the Error is not there.

import Test from '../Test.vue'

describe('run 2 test', () => {
  it('The error only appears if I run a test before I run the viewport change', () => {
    cy.mount(Test)
    cy.get('[data-testid="test1"]').get('a').should('contain', ' ..more')
  })

  it('The error originates on the first viewport changes it appears', () => {
    cy.mount(Test)
//so it appears here
    cy.viewport(550, 750)

    let charCount = 0
    cy.get('[data-testid="test1"]').then(($span) => {
      charCount = String($span.text()).length;
    })
    cy.viewport(400, 400)
    cy.get('[data-testid="test1"]').then(($span) => {
      const newCharCount = String($span.text()).length;
      expect(newCharCount).to.be.eq(charCount)
    })
  })
})

I found this anwser to the question: "Vue3 TypeError ref.value is null" and tried adding defineExpose() yet it doesn't change a thing. I tried re-declaring the const a = ref(null) within the onMounted() also no luck. Been searching but can't find a solution any help would be appreciated since a lot of the test will revolve around viewport changes.

The error causes the rest of the application, which depends on the a.value to fail.

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