Content in default slot causes component to re-render when the parent re-renders

[JosephSilber]

Version

2.5.16

Reproduction link

https://codepen.io/JosephSilber/pen/KeWjzr

Description

Passing content in a slot causes the component to re-render when the parent re-renders, even if the component's data hasn't changed.

Steps to reproduce

1. Create a simple component that shows the amount of times it's been rendered:

   Vue.component('times-rendered', {
       created() {
         this.timesRendered = 0
       },
       render(h) {        
         return h('div', `Times rendered: ${++this.timesRendered}`)
       }
   });

2. Use this component twice. Once with content in the default slot, once without. After the component is mounted, trigger any update to its data:

   <template>
       <div>
         <div>{{ updateTrigger }}</div>
         <times-rendered></times-rendered>
         <times-rendered>
           Any content in the default slot causes a re-render. Why?
         </times-rendered>
       </div>
   </template>
   
   <script>
   export default {
       data: () => ({ updateTrigger: false }),
       
       async mounted() {
         await this.$nextTick()
         
         this.updateTrigger = true
       },
   };
   </script>

What is expected?

I would expect each instance of the component to only render once.

What is actually happening?

The component with content in the slot renders twice.

---

I'm not actually sure whether this is a bug or a feature request. Maybe this is expected behavior currently. If it is, then I guess this would be better classified as a feature request.

[jacekkarczmarczyk]

Duplicate of #6351?

[posva]

The rerender happens because the whole template renders again because of updateTrigger
Let's mark this as a duplicate of #6351