view of replaced item is still in visible area

I want to replace the first item or an array (with different keys),
but the view of replaced item doesn't disappear from visible area.
they both show at position `0px `

```
# These don't work
Vue.set(app.items, 0, new_item)
app.$set(app.items, 0, new_item)
app.items.splice(idx, 1, new_item)

# This does work
app.items.shift()
setTimeout(function() {
app.items.unshift(new_item)
}, 0)
```