use watchEffect should pauseTracking with array

[okbug]

Version

2.7.0-beta.3

Reproduction link

codesandbox.io

Steps to reproduce

pauseTracking with array length when length-altering mutaion

What is expected?

the first demo in Vue3 can show the [ 1, 2 ]
but in Vue2.7.0-beta.3 it shows [ 1, 2, 1, 2 ]

In the second demo will make page stop running.
it looks like vuejs/core#2137

What is actually happening?

not same result and page stop running

[okbug]

the code looks like

<div id="demo">
        {{ arr }}
      </div>    

// it use vue2.7.0-beta3
<script src="../../dist/vue.min.js"></script>

<script>
const { ref, watchEffect, watch } = Vue
new Vue({
  setup() {
    const arr = ref([])
    watchEffect(() => {
        arr.value.push(1)
    })

    watchEffect(() => {
        arr.value.push(2)
    })

    return {
        arr
    }
  }
}).$mount('#demo')
</script>


<script>
    const arr = ref([])

    watchEffect(() => {
        arr.value.push(1)
    })
    watchEffect(() => {
        arr.value.push(2)
    })

    console.log(arr.value)
</script>

[yyx990803]

Unfortunately this is a limitation of Vue 2's reactivity system. Array mutation methods work by triggering updates of effects that tracks the array itself. There's no way to fix this.