vue.runtime.esm.js:619 [Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value. Prop being mutated: "placement"

原因是element ui的版本导致的问题

"element-ui": "^2.15.6",

改成

"element-ui": "^2.15.8",