size change not detected despite sizeDependencies

My rows have textarea, which can be dragged to resize
I'm using mutationObserver to detect the size change and update data on the component
and I'm passing this data field as the sizeDependencies (ie. `:sizeDependencies="['textAreaHeight']"`),

but despite the data getting modified, the size isn't recalculated.

I tried to use also the `IdState` (see below) and `:sizeDependencies="['idState.textAreaHeight']"` but had same affect.

Can you help me detect my error?

```javascript
export default {
name: 'MockRow',
extends: DynamicScrollerItem,
idState() { //replaces data - see https://github.com/Akryum/vue-virtual-scroller#idstate
return {
textAreaHeight: -1
}
},
mixins: [
IdState({
idProp: vm => vm.item.name,
}),
],
mounted(){
let textarea = this.$refs.textarea;
let $this=this;
function outputsize() {
console.log( textarea.offsetHeight)
$this.idState.textAreaHeight = textarea.offsetHeight;
}
outputsize();
this.areaChangeObserver = new MutationObserver(outputsize).observe(textarea, {
attributes: true, attributeFilter: [ "style" ]
})
},
beforeDestroy() {
if (this.areaChangeObserver) {
this.areaChangeObserver.disconnect();
}
},
```