[Bug Report] double scroll div box caused the el-select dropdown menu position invalid

[luguosheng1314]

Element UI version

1.2.0

OS/Browsers version

chrome

Vue version

2.1.0

Reproduction Link

https://jsfiddle.net/hrw1m7yq/

Steps to reproduce

click the el-select component to show menu and then rolling the div scrollbar

What is Expected?

the menu would move with the el-select

What is actually happening?

the menu can not move with the el-select

[qingdengyue]

Like HTML select .It should close the dropdown list when click the ScrollBar. That is a good way.

[luguosheng1314]

if user use touch mouse in mac to roll the div scrollbar, this appearance looks strange