onMouseDown causes splash on real ios browsers

<!-- Note: if the issue is about documentation or the website, please file it at: https://github.com/reactjs/reactjs.org/issues/new --> **Do you want to request a *feature* or report a *bug*?** *bug* **What is the current behavior?** unexpected **grey splash** caused by `onMouseDown` on real ios device's browsers, both safari and chrome. [Please checkout this **grey splash** issue first](https://github.com/mui-org/material-ui/issues/11154) I did some trial later, and find out that vanilla html `<button onmousedown="">btn</button>` doesn't cause **grey splash**, but with React's jsx `<button onMouseDown={()=>{}}></button>`, there is a noticeable **grey splash** **What is the expected behavior?** who wants a splash? **Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?** "react": "^16.3.2"